diff --git a/composer.lock b/composer.lock
index b645632..3b54d14 100644
--- a/composer.lock
+++ b/composer.lock
@@ -3078,6 +3078,42 @@
             "time": "2016-07-05T20:48:03+00:00"
         },
         {
+            "name": "drupal/coder",
+            "version": "8.2.8",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/klausi/coder.git",
+                "reference": "6d717e1a5a5dd592ebbeaafad11746849fb52532"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/klausi/coder/zipball/6d717e1a5a5dd592ebbeaafad11746849fb52532",
+                "reference": "6d717e1a5a5dd592ebbeaafad11746849fb52532",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=5.4.0",
+                "squizlabs/php_codesniffer": ">=2.5.1",
+                "symfony/yaml": ">=2.0.0"
+            },
+            "require-dev": {
+                "phpunit/phpunit": ">=3.7"
+            },
+            "type": "library",
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "GPL-2.0+"
+            ],
+            "description": "Coder is a library to review Drupal code.",
+            "homepage": "https://www.drupal.org/project/coder",
+            "keywords": [
+                "code review",
+                "phpcs",
+                "standards"
+            ],
+            "time": "2016-07-05T20:48:03+00:00"
+        },
+        {
             "name": "fabpot/goutte",
             "version": "v3.1.2",
             "source": {
@@ -4217,6 +4253,84 @@
             "time": "2016-11-30T04:02:31+00:00"
         },
         {
+            "name": "squizlabs/php_codesniffer",
+            "version": "2.7.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/squizlabs/PHP_CodeSniffer.git",
+                "reference": "9b324f3a1132459a7274a0ace2e1b766ba80930f"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/squizlabs/PHP_CodeSniffer/zipball/9b324f3a1132459a7274a0ace2e1b766ba80930f",
+                "reference": "9b324f3a1132459a7274a0ace2e1b766ba80930f",
+                "shasum": ""
+            },
+            "require": {
+                "ext-simplexml": "*",
+                "ext-tokenizer": "*",
+                "ext-xmlwriter": "*",
+                "php": ">=5.1.2"
+            },
+            "require-dev": {
+                "phpunit/phpunit": "~4.0"
+            },
+            "bin": [
+                "scripts/phpcs",
+                "scripts/phpcbf"
+            ],
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "2.x-dev"
+                }
+            },
+            "autoload": {
+                "classmap": [
+                    "CodeSniffer.php",
+                    "CodeSniffer/CLI.php",
+                    "CodeSniffer/Exception.php",
+                    "CodeSniffer/File.php",
+                    "CodeSniffer/Fixer.php",
+                    "CodeSniffer/Report.php",
+                    "CodeSniffer/Reporting.php",
+                    "CodeSniffer/Sniff.php",
+                    "CodeSniffer/Tokens.php",
+                    "CodeSniffer/Reports/",
+                    "CodeSniffer/Tokenizers/",
+                    "CodeSniffer/DocGenerators/",
+                    "CodeSniffer/Standards/AbstractPatternSniff.php",
+                    "CodeSniffer/Standards/AbstractScopeSniff.php",
+                    "CodeSniffer/Standards/AbstractVariableSniff.php",
+                    "CodeSniffer/Standards/IncorrectPatternException.php",
+                    "CodeSniffer/Standards/Generic/Sniffs/",
+                    "CodeSniffer/Standards/MySource/Sniffs/",
+                    "CodeSniffer/Standards/PEAR/Sniffs/",
+                    "CodeSniffer/Standards/PSR1/Sniffs/",
+                    "CodeSniffer/Standards/PSR2/Sniffs/",
+                    "CodeSniffer/Standards/Squiz/Sniffs/",
+                    "CodeSniffer/Standards/Zend/Sniffs/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "BSD-3-Clause"
+            ],
+            "authors": [
+                {
+                    "name": "Greg Sherwood",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP_CodeSniffer tokenizes PHP, JavaScript and CSS files and detects violations of a defined set of coding standards.",
+            "homepage": "http://www.squizlabs.com/php-codesniffer",
+            "keywords": [
+                "phpcs",
+                "standards"
+            ],
+            "time": "2016-11-30T04:02:31+00:00"
+        },
+        {
             "name": "symfony/browser-kit",
             "version": "v2.8.16",
             "source": {
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 687a62c..5832f12 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -13,6 +13,7 @@
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Url;
 use Drupal\file\Entity\File;
+use Drupal\file\FileFormTrait;
 use Drupal\file\FileInterface;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\Unicode;
@@ -710,206 +711,20 @@ function file_cron() {
  *   An array of file entities or a single file entity if $delta != NULL. Each
  *   array element contains the file entity if the upload succeeded or FALSE if
  *   there was an error. Function returns NULL if no file was uploaded.
+ *
+ * @deprecated in Drupal 8.3.x, will be removed before Drupal 9.0.0.
+ *   This function should not be used for form validation, use
+ *   \Drupal\file\FileFormTrait::fileSaveUpload() instead.
+ *
+ * @see \Drupal\file\FileFormTrait::fileSaveUpload()
  */
 function file_save_upload($form_field_name, $validators = array(), $destination = FALSE, $delta = NULL, $replace = FILE_EXISTS_RENAME) {
-  $user = \Drupal::currentUser();
-  static $upload_cache;
-
-  $all_files = \Drupal::request()->files->get('files', array());
-  // Make sure there's an upload to process.
-  if (empty($all_files[$form_field_name])) {
-    return NULL;
-  }
-  $file_upload = $all_files[$form_field_name];
-
-  // Return cached objects without processing since the file will have
-  // already been processed and the paths in $_FILES will be invalid.
-  if (isset($upload_cache[$form_field_name])) {
-    if (isset($delta)) {
-      return $upload_cache[$form_field_name][$delta];
-    }
-    return $upload_cache[$form_field_name];
-  }
-
-  // Prepare uploaded files info. Representation is slightly different
-  // for multiple uploads and we fix that here.
-  $uploaded_files = $file_upload;
-  if (!is_array($file_upload)) {
-    $uploaded_files = array($file_upload);
-  }
-
-  $files = array();
-  foreach ($uploaded_files as $i => $file_info) {
-    // Check for file upload errors and return FALSE for this file if a lower
-    // level system error occurred. For a complete list of errors:
-    // See http://php.net/manual/features.file-upload.errors.php.
-    switch ($file_info->getError()) {
-      case UPLOAD_ERR_INI_SIZE:
-      case UPLOAD_ERR_FORM_SIZE:
-        drupal_set_message(t('The file %file could not be saved because it exceeds %maxsize, the maximum allowed size for uploads.', array('%file' => $file_info->getFilename(), '%maxsize' => format_size(file_upload_max_size()))), 'error');
-        $files[$i] = FALSE;
-        continue;
-
-      case UPLOAD_ERR_PARTIAL:
-      case UPLOAD_ERR_NO_FILE:
-        drupal_set_message(t('The file %file could not be saved because the upload did not complete.', array('%file' => $file_info->getFilename())), 'error');
-        $files[$i] = FALSE;
-        continue;
-
-      case UPLOAD_ERR_OK:
-        // Final check that this is a valid upload, if it isn't, use the
-        // default error handler.
-        if (is_uploaded_file($file_info->getRealPath())) {
-          break;
-        }
-
-        // Unknown error
-      default:
-        drupal_set_message(t('The file %file could not be saved. An unknown error has occurred.', array('%file' => $file_info->getFilename())), 'error');
-        $files[$i] = FALSE;
-        continue;
-
-    }
-    // Begin building file entity.
-    $values = array(
-      'uid' => $user->id(),
-      'status' => 0,
-      'filename' => $file_info->getClientOriginalName(),
-      'uri' => $file_info->getRealPath(),
-      'filesize' => $file_info->getSize(),
-    );
-    $values['filemime'] = \Drupal::service('file.mime_type.guesser')->guess($values['filename']);
-    $file = File::create($values);
-
-    $extensions = '';
-    if (isset($validators['file_validate_extensions'])) {
-      if (isset($validators['file_validate_extensions'][0])) {
-        // Build the list of non-munged extensions if the caller provided them.
-        $extensions = $validators['file_validate_extensions'][0];
-      }
-      else {
-        // If 'file_validate_extensions' is set and the list is empty then the
-        // caller wants to allow any extension. In this case we have to remove the
-        // validator or else it will reject all extensions.
-        unset($validators['file_validate_extensions']);
-      }
-    }
-    else {
-      // No validator was provided, so add one using the default list.
-      // Build a default non-munged safe list for file_munge_filename().
-      $extensions = 'jpg jpeg gif png txt doc xls pdf ppt pps odt ods odp';
-      $validators['file_validate_extensions'] = array();
-      $validators['file_validate_extensions'][0] = $extensions;
-    }
-
-    if (!empty($extensions)) {
-      // Munge the filename to protect against possible malicious extension
-      // hiding within an unknown file type (ie: filename.html.foo).
-      $file->setFilename(file_munge_filename($file->getFilename(), $extensions));
-    }
-
-    // Rename potentially executable files, to help prevent exploits (i.e. will
-    // rename filename.php.foo and filename.php to filename.php.foo.txt and
-    // filename.php.txt, respectively). Don't rename if 'allow_insecure_uploads'
-    // evaluates to TRUE.
-    if (!\Drupal::config('system.file')->get('allow_insecure_uploads') && preg_match('/\.(php|pl|py|cgi|asp|js)(\.|$)/i', $file->getFilename()) && (substr($file->getFilename(), -4) != '.txt')) {
-      $file->setMimeType('text/plain');
-      // The destination filename will also later be used to create the URI.
-      $file->setFilename($file->getFilename() . '.txt');
-      // The .txt extension may not be in the allowed list of extensions. We have
-      // to add it here or else the file upload will fail.
-      if (!empty($extensions)) {
-        $validators['file_validate_extensions'][0] .= ' txt';
-        drupal_set_message(t('For security reasons, your upload has been renamed to %filename.', array('%filename' => $file->getFilename())));
-      }
-    }
-
-    // If the destination is not provided, use the temporary directory.
-    if (empty($destination)) {
-      $destination = 'temporary://';
-    }
-
-    // Assert that the destination contains a valid stream.
-    $destination_scheme = file_uri_scheme($destination);
-    if (!file_stream_wrapper_valid_scheme($destination_scheme)) {
-      drupal_set_message(t('The file could not be uploaded because the destination %destination is invalid.', array('%destination' => $destination)), 'error');
-      $files[$i] = FALSE;
-      continue;
-    }
-
-    $file->source = $form_field_name;
-    // A file URI may already have a trailing slash or look like "public://".
-    if (substr($destination, -1) != '/') {
-      $destination .= '/';
-    }
-    $file->destination = file_destination($destination . $file->getFilename(), $replace);
-    // If file_destination() returns FALSE then $replace === FILE_EXISTS_ERROR and
-    // there's an existing file so we need to bail.
-    if ($file->destination === FALSE) {
-      drupal_set_message(t('The file %source could not be uploaded because a file by that name already exists in the destination %directory.', array('%source' => $form_field_name, '%directory' => $destination)), 'error');
-      $files[$i] = FALSE;
-      continue;
-    }
-
-    // Add in our check of the file name length.
-    $validators['file_validate_name_length'] = array();
-
-    // Call the validation functions specified by this function's caller.
-    $errors = file_validate($file, $validators);
-
-    // Check for errors.
-    if (!empty($errors)) {
-      $message = array(
-        'error' => array(
-          '#markup' => t('The specified file %name could not be uploaded.', array('%name' => $file->getFilename())),
-        ),
-        'item_list' => array(
-          '#theme' => 'item_list',
-          '#items' => $errors,
-        ),
-      );
-      // @todo Add support for render arrays in drupal_set_message()? See
-      //  https://www.drupal.org/node/2505497.
-      drupal_set_message(\Drupal::service('renderer')->renderPlain($message), 'error');
-      $files[$i] = FALSE;
-      continue;
-    }
-
-    // Move uploaded files from PHP's upload_tmp_dir to Drupal's temporary
-    // directory. This overcomes open_basedir restrictions for future file
-    // operations.
-    $file->setFileUri($file->destination);
-    if (!drupal_move_uploaded_file($file_info->getRealPath(), $file->getFileUri())) {
-      drupal_set_message(t('File upload error. Could not move uploaded file.'), 'error');
-      \Drupal::logger('file')->notice('Upload error. Could not move uploaded file %file to destination %destination.', array('%file' => $file->getFilename(), '%destination' => $file->getFileUri()));
-      $files[$i] = FALSE;
-      continue;
-    }
-
-    // Set the permissions on the new file.
-    drupal_chmod($file->getFileUri());
-
-    // If we are replacing an existing file re-use its database record.
-    // @todo Do not create a new entity in order to update it. See
-    //   https://www.drupal.org/node/2241865.
-    if ($replace == FILE_EXISTS_REPLACE) {
-      $existing_files = entity_load_multiple_by_properties('file', array('uri' => $file->getFileUri()));
-      if (count($existing_files)) {
-        $existing = reset($existing_files);
-        $file->fid = $existing->id();
-        $file->setOriginalId($existing->id());
-      }
-    }
-
-    // If we made it this far it's safe to record this file in the database.
-    $file->save();
-    $files[$i] = $file;
+  $errors = [];
+  $files = FileFormTrait::_fileSaveUpload($form_field_name, $validators, $destination, $delta, $replace, $errors);
+  foreach ($errors as $error) {
+    drupal_set_message($error, 'error');
   }
-
-  // Add files to the cache.
-  $upload_cache[$form_field_name] = $files;
-
-  return isset($delta) ? $files[$delta] : $files;
+  return $files;
 }
 
 /**
@@ -1192,9 +1007,8 @@ function file_managed_file_save_upload($element, FormStateInterface $form_state)
   $files_uploaded = $element['#multiple'] && count(array_filter($file_upload)) > 0;
   $files_uploaded |= !$element['#multiple'] && !empty($file_upload);
   if ($files_uploaded) {
-    if (!$files = file_save_upload($upload_name, $element['#upload_validators'], $destination)) {
+    if (!$files = FileFormTrait::fileSaveUpload($element, $form_state)) {
       \Drupal::logger('file')->notice('The file upload failed. %upload', array('%upload' => $upload_name));
-      $form_state->setError($element, t('Files in the @name field were unable to be uploaded.', array('@name' => $element['#title'])));
       return array();
     }
 
diff --git a/core/modules/file/src/FileFormTrait.php b/core/modules/file/src/FileFormTrait.php
new file mode 100644
index 0000000..1554e84
--- /dev/null
+++ b/core/modules/file/src/FileFormTrait.php
@@ -0,0 +1,317 @@
+<?php
+
+namespace Drupal\file;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\file\Entity\File;
+
+/**
+ * Adds methods to save files to forms.
+ */
+trait FileFormTrait {
+
+  /**
+   * Saves form file uploads.
+   *
+   * The files will be added to the {file_managed} table as temporary files.
+   * Temporary files are periodically cleaned. Use the 'file.usage' service to
+   * register the usage of the file which will automatically mark it as permanent.
+   *
+   * @param array $element
+   *   The FAPI element whose values are being saved.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param null|int $delta
+   *   (optional) The delta of the file to return the file entity.
+   *   Defaults to NULL.
+   * @param int $replace
+   *   (optional) The replace behavior when the destination file already exists.
+   *   Possible values include:
+   *   - FILE_EXISTS_REPLACE: Replace the existing file.
+   *   - FILE_EXISTS_RENAME: (default) Append _{incrementing number} until the
+   *     filename is unique.
+   *   - FILE_EXISTS_ERROR: Do nothing and return FALSE.
+   *
+   * @return array|\Drupal\file\FileInterface|null|false
+   *   An array of file entities or a single file entity if $delta != NULL. Each
+   *   array element contains the file entity if the upload succeeded or FALSE if
+   *   there was an error. Function returns NULL if no file was uploaded.
+   */
+  public static function fileSaveUpload($element, FormStateInterface $form_state, $delta = NULL, $replace = FILE_EXISTS_RENAME) {
+    $upload_location = isset($element['#upload_location']) ? $element['#upload_location'] : FALSE;
+    $upload_name = implode('_', $element['#parents']);
+    $upload_validators = isset($element['#upload_validators']) ? $element['#upload_validators'] : [];
+    $errors = [];
+    $files = static::_fileSaveUpload($upload_name, $upload_validators, $upload_location, $delta, $replace, $errors);
+
+    if (!empty($errors)) {
+      if (count($errors) > 1) {
+        // Render multiple errors into a single message.
+        $render_array = [
+          'error' => [
+            '#markup' => t('One or more files could not be uploaded.'),
+          ],
+          'item_list' => [
+            '#theme' => 'item_list',
+            '#items' => $errors,
+          ],
+        ];
+        $error_message = \Drupal::service('renderer')->renderPlain($render_array);
+      }
+      else {
+        $error_message = reset($errors);
+      }
+      $form_state->setError($element, $error_message);
+    }
+
+    return $files;
+  }
+
+  /**
+   * Saves file uploads to a new location.
+   *
+   * The files will be added to the {file_managed} table as temporary files.
+   * Temporary files are periodically cleaned. Use the 'file.usage' service to
+   * register the usage of the file which will automatically mark it as permanent.
+   *
+   * @param string $form_field_name
+   *   A string that is the associative array key of the upload form element in
+   *   the form array.
+   * @param array $validators
+   *   (optional) An associative array of callback functions used to validate the
+   *   file. See file_validate() for a full discussion of the array format.
+   *   If the array is empty, it will be set up to call file_validate_extensions()
+   *   with a safe list of extensions, as follows: "jpg jpeg gif png txt doc
+   *   xls pdf ppt pps odt ods odp". To allow all extensions, you must explicitly
+   *   set this array to ['file_validate_extensions' => '']. (Beware: this is not
+   *   safe and should only be allowed for trusted users, if at all.)
+   * @param string|false $destination
+   *   (optional) A string containing the URI that the file should be copied to.
+   *   This must be a stream wrapper URI. If this value is omitted or set to
+   *   FALSE, Drupal's temporary files scheme will be used ("temporary://").
+   * @param null|int $delta
+   *   (optional) The delta of the file to return the file entity.
+   *   Defaults to NULL.
+   * @param int $replace
+   *   (optional) The replace behavior when the destination file already exists.
+   *   Possible values include:
+   *   - FILE_EXISTS_REPLACE: Replace the existing file.
+   *   - FILE_EXISTS_RENAME: (default) Append _{incrementing number} until the
+   *     filename is unique.
+   *   - FILE_EXISTS_ERROR: Do nothing and return FALSE.
+   * @param array $errors_to_return
+   *   (optional) Any errors that have occurred whilst uploading the file.
+   *
+   * @return array|\Drupal\file\FileInterface|null|false
+   *   An array of file entities or a single file entity if $delta != NULL. Each
+   *   array element contains the file entity if the upload succeeded or FALSE if
+   *   there was an error. Function returns NULL if no file was uploaded.
+   *
+   * @see file_save_upload()
+   * @see file_save_upload()
+   *
+   * @internal
+   */
+  public static function _fileSaveUpload($form_field_name, $validators = [], $destination = FALSE, $delta = NULL, $replace = FILE_EXISTS_RENAME, &$errors_to_return = []) {
+    $user = \Drupal::currentUser();
+    static $upload_cache;
+
+    $all_files = \Drupal::request()->files->get('files', []);
+    // Make sure there's an upload to process.
+    if (empty($all_files[$form_field_name])) {
+      return NULL;
+    }
+    $file_upload = $all_files[$form_field_name];
+
+    // Return cached objects without processing since the file will have
+    // already been processed and the paths in $_FILES will be invalid.
+    if (isset($upload_cache[$form_field_name])) {
+      if (isset($delta)) {
+        return $upload_cache[$form_field_name][$delta];
+      }
+      return $upload_cache[$form_field_name];
+    }
+
+    // Prepare uploaded files info. Representation is slightly different
+    // for multiple uploads and we fix that here.
+    $uploaded_files = $file_upload;
+    if (!is_array($file_upload)) {
+      $uploaded_files = array($file_upload);
+    }
+
+    $files = array();
+    foreach ($uploaded_files as $i => $file_info) {
+      // Check for file upload errors and return FALSE for this file if a lower
+      // level system error occurred. For a complete list of errors:
+      // See http://php.net/manual/features.file-upload.errors.php.
+      switch ($file_info->getError()) {
+        case UPLOAD_ERR_INI_SIZE:
+        case UPLOAD_ERR_FORM_SIZE:
+          $errors_to_return[] = new TranslatableMarkup('The file %file could not be saved because it exceeds %maxsize, the maximum allowed size for uploads.', ['%file' => $file_info->getFilename(), '%maxsize' => format_size(file_upload_max_size())]);
+          $files[$i] = FALSE;
+          break;
+
+        case UPLOAD_ERR_PARTIAL:
+        case UPLOAD_ERR_NO_FILE:
+          $errors_to_return[] = new TranslatableMarkup('The file %file could not be saved because the upload did not complete.', ['%file' => $file_info->getFilename()]);
+          $files[$i] = FALSE;
+          break;
+
+        case UPLOAD_ERR_OK:
+          // Final check that this is a valid upload, if it isn't, use the
+          // default error handler.
+          if (is_uploaded_file($file_info->getRealPath())) {
+            break;
+          }
+
+        default:
+          // Unknown error.
+          $errors_to_return[] = new TranslatableMarkup('The file %file could not be saved. An unknown error has occurred.', ['%file' => $file_info->getFilename()]);
+          $files[$i] = FALSE;
+          break;
+
+      }
+      // Begin building file entity.
+      $values = [
+        'uid' => $user->id(),
+        'status' => 0,
+        'filename' => $file_info->getClientOriginalName(),
+        'uri' => $file_info->getRealPath(),
+        'filesize' => $file_info->getSize(),
+      ];
+      $values['filemime'] = \Drupal::service('file.mime_type.guesser')->guess($values['filename']);
+      $file = File::create($values);
+
+      $extensions = '';
+      if (isset($validators['file_validate_extensions'])) {
+        if (isset($validators['file_validate_extensions'][0])) {
+          // Build the list of non-munged extensions if the caller provided them.
+          $extensions = $validators['file_validate_extensions'][0];
+        }
+        else {
+          // If 'file_validate_extensions' is set and the list is empty then the
+          // caller wants to allow any extension. In this case we have to remove the
+          // validator or else it will reject all extensions.
+          unset($validators['file_validate_extensions']);
+        }
+      }
+      else {
+        // No validator was provided, so add one using the default list.
+        // Build a default non-munged safe list for file_munge_filename().
+        $extensions = 'jpg jpeg gif png txt doc xls pdf ppt pps odt ods odp';
+        $validators['file_validate_extensions'] = array();
+        $validators['file_validate_extensions'][0] = $extensions;
+      }
+
+      if (!empty($extensions)) {
+        // Munge the filename to protect against possible malicious extension
+        // hiding within an unknown file type (ie: filename.html.foo).
+        $file->setFilename(file_munge_filename($file->getFilename(), $extensions));
+      }
+
+      // Rename potentially executable files, to help prevent exploits (i.e. will
+      // rename filename.php.foo and filename.php to filename.php.foo.txt and
+      // filename.php.txt, respectively). Don't rename if 'allow_insecure_uploads'
+      // evaluates to TRUE.
+      if (!\Drupal::config('system.file')->get('allow_insecure_uploads') && preg_match('/\.(php|pl|py|cgi|asp|js)(\.|$)/i', $file->getFilename()) && (substr($file->getFilename(), -4) != '.txt')) {
+        $file->setMimeType('text/plain');
+        // The destination filename will also later be used to create the URI.
+        $file->setFilename($file->getFilename() . '.txt');
+        // The .txt extension may not be in the allowed list of extensions. We have
+        // to add it here or else the file upload will fail.
+        if (!empty($extensions)) {
+          $validators['file_validate_extensions'][0] .= ' txt';
+          drupal_set_message(new TranslatableMarkup('For security reasons, your upload has been renamed to %filename.', ['%filename' => $file->getFilename()]));
+        }
+      }
+
+      // If the destination is not provided, use the temporary directory.
+      if (empty($destination)) {
+        $destination = 'temporary://';
+      }
+
+      // Assert that the destination contains a valid stream.
+      $destination_scheme = file_uri_scheme($destination);
+      if (!file_stream_wrapper_valid_scheme($destination_scheme)) {
+        $errors_to_return[] = new TranslatableMarkup('The file could not be uploaded because the destination %destination is invalid.', ['%destination' => $destination]);
+        $files[$i] = FALSE;
+        continue;
+      }
+
+      $file->source = $form_field_name;
+      // A file URI may already have a trailing slash or look like "public://".
+      if (substr($destination, -1) != '/') {
+        $destination .= '/';
+      }
+      $file->destination = file_destination($destination . $file->getFilename(), $replace);
+      // If file_destination() returns FALSE then $replace === FILE_EXISTS_ERROR and
+      // there's an existing file so we need to bail.
+      if ($file->destination === FALSE) {
+        $errors_to_return[] = new TranslatableMarkup('The file %source could not be uploaded because a file by that name already exists in the destination %directory.', ['%source' => $form_field_name, '%directory' => $destination]);
+        $files[$i] = FALSE;
+        continue;
+      }
+
+      // Add in our check of the file name length.
+      $validators['file_validate_name_length'] = array();
+
+      // Call the validation functions specified by this function's caller.
+      $errors = file_validate($file, $validators);
+
+      // Check for errors.
+      if (!empty($errors)) {
+        $message = array(
+          'error' => array(
+            '#markup' => t('The specified file %name could not be uploaded.', array('%name' => $file->getFilename())),
+          ),
+          'item_list' => array(
+            '#theme' => 'item_list',
+            '#items' => $errors,
+          ),
+        );
+        // @todo Add support for render arrays in drupal_set_message()? See
+        //  https://www.drupal.org/node/2505497.
+        $errors_to_return[] = \Drupal::service('renderer')->renderPlain($message);
+        $files[$i] = FALSE;
+        continue;
+      }
+
+      // Move uploaded files from PHP's upload_tmp_dir to Drupal's temporary
+      // directory. This overcomes open_basedir restrictions for future file
+      // operations.
+      $file->setFileUri($file->destination);
+      if (!\Drupal::service('file_system')->moveUploadedFile($file_info->getRealPath(), $file->getFileUri())) {
+        $errors_to_return[] = new TranslatableMarkup('File upload error. Could not move uploaded file.');
+        \Drupal::logger('file')->notice('Upload error. Could not move uploaded file %file to destination %destination.', array('%file' => $file->getFilename(), '%destination' => $file->getFileUri()));
+        $files[$i] = FALSE;
+        continue;
+      }
+
+      // Set the permissions on the new file.
+      drupal_chmod($file->getFileUri());
+
+      // If we are replacing an existing file re-use its database record.
+      // @todo Do not create a new entity in order to update it. See
+      //   https://www.drupal.org/node/2241865.
+      if ($replace == FILE_EXISTS_REPLACE) {
+        $existing_files = entity_load_multiple_by_properties('file', array('uri' => $file->getFileUri()));
+        if (count($existing_files)) {
+          $existing = reset($existing_files);
+          $file->fid = $existing->id();
+          $file->setOriginalId($existing->id());
+        }
+      }
+
+      // If we made it this far it's safe to record this file in the database.
+      $file->save();
+      $files[$i] = $file;
+    }
+
+    // Add files to the cache.
+    $upload_cache[$form_field_name] = $files;
+
+    return isset($delta) ? $files[$delta] : $files;
+  }
+
+}
diff --git a/core/modules/file/src/Tests/SaveUploadFormTest.php b/core/modules/file/src/Tests/SaveUploadFormTest.php
new file mode 100644
index 0000000..3714b97
--- /dev/null
+++ b/core/modules/file/src/Tests/SaveUploadFormTest.php
@@ -0,0 +1,460 @@
+<?php
+
+namespace Drupal\file\Tests;
+
+use Drupal\file\Entity\File;
+
+/**
+ * Tests the \Drupal\file\FileFormTrait::fileSaveUpload() function.
+ *
+ * @group file
+ *
+ * @see \Drupal\file\FileFormTrait::fileSaveUpload()
+ */
+class SaveUploadFormTest extends FileManagedTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['dblog'];
+
+  /**
+   * An image file path for uploading.
+   *
+   * @var \Drupal\file\FileInterface
+   */
+  protected $image;
+
+  /**
+   * A PHP file path for upload security testing.
+   */
+  protected $phpfile;
+
+  /**
+   * The largest file id when the test starts.
+   */
+  protected $maxFidBefore;
+
+  /**
+   * Extension of the image filename.
+   *
+   * @var string
+   */
+  protected $imageExtension;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $account = $this->drupalCreateUser(['access site reports']);
+    $this->drupalLogin($account);
+
+    $image_files = $this->drupalGetTestFiles('image');
+    $this->image = File::create((array) current($image_files));
+
+    list(, $this->imageExtension) = explode('.', $this->image->getFilename());
+    $this->assertTrue(is_file($this->image->getFileUri()), "The image file we're going to upload exists.");
+
+    $this->phpfile = current($this->drupalGetTestFiles('php'));
+    $this->assertTrue(is_file($this->phpfile->uri), 'The PHP file we are going to upload exists.');
+
+    $this->maxFidBefore = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
+
+    // Upload with replace to guarantee there's something there.
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called then clean out the hook
+    // counters.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+    file_test_reset();
+  }
+
+  /**
+   * Tests the file_save_upload_from_form() function.
+   */
+  public function testNormal() {
+    $max_fid_after = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
+    $this->assertTrue($max_fid_after > $this->maxFidBefore, 'A new file was created.');
+    $file1 = File::load($max_fid_after);
+    $this->assertTrue($file1, 'Loaded the file.');
+    // MIME type of the uploaded image may be either image/jpeg or image/png.
+    $this->assertEqual(substr($file1->getMimeType(), 0, 5), 'image', 'A MIME type was set.');
+
+    // Reset the hook counters to get rid of the 'load' we just called.
+    file_test_reset();
+
+    // Upload a second file.
+    $image2 = current($this->drupalGetTestFiles('image'));
+    $edit = ['files[file_test_upload][]' => drupal_realpath($image2->uri)];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'));
+    $max_fid_after = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+
+    $file2 = File::load($max_fid_after);
+    $this->assertTrue($file2, 'Loaded the file');
+    // MIME type of the uploaded image may be either image/jpeg or image/png.
+    $this->assertEqual(substr($file2->getMimeType(), 0, 5), 'image', 'A MIME type was set.');
+
+    // Load both files using File::loadMultiple().
+    $files = File::loadMultiple([$file1->id(), $file2->id()]);
+    $this->assertTrue(isset($files[$file1->id()]), 'File was loaded successfully');
+    $this->assertTrue(isset($files[$file2->id()]), 'File was loaded successfully');
+
+    // Upload a third file to a subdirectory.
+    $image3 = current($this->drupalGetTestFiles('image'));
+    $image3_realpath = drupal_realpath($image3->uri);
+    $dir = $this->randomMachineName();
+    $edit = [
+      'files[file_test_upload][]' => $image3_realpath,
+      'file_subdir' => $dir,
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'));
+    $this->assertTrue(is_file('temporary://' . $dir . '/' . trim(drupal_basename($image3_realpath))));
+  }
+
+  /**
+   * Tests extension handling.
+   */
+  public function testHandleExtension() {
+    // The file being tested is a .gif which is in the default safe list
+    // of extensions to allow when the extension validator isn't used. This is
+    // implicitly tested at the testNormal() test. Here we tell
+    // file_save_upload_from_form() to only allow ".foo".
+    $extensions = 'foo';
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'extensions' => $extensions,
+    ];
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $message = t('Only files with the following extensions are allowed:') . ' <em class="placeholder">' . $extensions . '</em>';
+    $this->assertRaw($message, 'Cannot upload a disallowed extension');
+    $this->assertRaw(t('Epic upload FAIL!'), 'Found the failure message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate']);
+
+    // Reset the hook counters.
+    file_test_reset();
+
+    $extensions = 'foo ' . $this->imageExtension;
+    // Now tell file_save_upload_from_form() to allow the extension of our test image.
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'extensions' => $extensions,
+    ];
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertNoRaw(t('Only files with the following extensions are allowed:'), 'Can upload an allowed extension.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'load', 'update']);
+
+    // Reset the hook counters.
+    file_test_reset();
+
+    // Now tell file_save_upload_from_form() to allow any extension.
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'allow_all_extensions' => TRUE,
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertNoRaw(t('Only files with the following extensions are allowed:'), 'Can upload any extension.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'load', 'update']);
+  }
+
+  /**
+   * Tests dangerous file handling.
+   */
+  public function testHandleDangerousFile() {
+    $config = $this->config('system.file');
+    // Allow the .php extension and make sure it gets renamed to .txt for
+    // safety. Also check to make sure its MIME type was changed.
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload][]' => drupal_realpath($this->phpfile->uri),
+      'is_image_file' => FALSE,
+      'extensions' => 'php',
+    ];
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $message = t('For security reasons, your upload has been renamed to') . ' <em class="placeholder">' . $this->phpfile->filename . '.txt' . '</em>';
+    $this->assertRaw($message, 'Dangerous file was renamed.');
+    $this->assertRaw(t('File MIME type is text/plain.'), "Dangerous file's MIME type was changed.");
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+
+    // Ensure dangerous files are not renamed when insecure uploads is TRUE.
+    // Turn on insecure uploads.
+    $config->set('allow_insecure_uploads', 1)->save();
+    // Reset the hook counters.
+    file_test_reset();
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertNoRaw(t('For security reasons, your upload has been renamed'), 'Found no security message.');
+    $this->assertRaw(t('File name is @filename', ['@filename' => $this->phpfile->filename]), 'Dangerous file was not renamed when insecure uploads is TRUE.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+
+    // Turn off insecure uploads.
+    $config->set('allow_insecure_uploads', 0)->save();
+  }
+
+  /**
+   * Tests file munge handling.
+   */
+  public function testHandleFileMunge() {
+    // Ensure insecure uploads are disabled for this test.
+    $this->config('system.file')->set('allow_insecure_uploads', 0)->save();
+    $this->image = file_move($this->image, $this->image->getFileUri() . '.foo.' . $this->imageExtension);
+
+    // Reset the hook counters to get rid of the 'move' we just called.
+    file_test_reset();
+
+    $extensions = $this->imageExtension;
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'extensions' => $extensions,
+    ];
+
+    $munged_filename = $this->image->getFilename();
+    $munged_filename = substr($munged_filename, 0, strrpos($munged_filename, '.'));
+    $munged_filename .= '_.' . $this->imageExtension;
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('For security reasons, your upload has been renamed'), 'Found security message.');
+    $this->assertRaw(t('File name is @filename', ['@filename' => $munged_filename]), 'File was successfully munged.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+
+    // Ensure we don't munge files if we're allowing any extension.
+    // Reset the hook counters.
+    file_test_reset();
+
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'allow_all_extensions' => TRUE,
+    ];
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertNoRaw(t('For security reasons, your upload has been renamed'), 'Found no security message.');
+    $this->assertRaw(t('File name is @filename', ['@filename' => $this->image->getFilename()]), 'File was not munged when allowing any extension.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+  }
+
+  /**
+   * Tests renaming when uploading over a file that already exists.
+   */
+  public function testExistingRename() {
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_RENAME,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'insert']);
+  }
+
+  /**
+   * Tests replacement when uploading over a file that already exists.
+   */
+  public function testExistingReplace() {
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(['validate', 'load', 'update']);
+  }
+
+  /**
+   * Tests for failure when uploading over a file that already exists.
+   */
+  public function testExistingError() {
+    $edit = [
+      'file_test_replace' => FILE_EXISTS_ERROR,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('Epic upload FAIL!'), 'Found the failure message.');
+
+    // Check that the no hooks were called while failing.
+    $this->assertFileHooksCalled([]);
+  }
+
+  /**
+   * Tests for no failures when not uploading a file.
+   */
+  public function testNoUpload() {
+    $this->drupalPostForm('file-test/save_upload_from_form_test', [], t('Submit'));
+    $this->assertNoRaw(t('Epic upload FAIL!'), 'Failure message not found.');
+  }
+
+  /**
+   * Tests for log entry on failing destination.
+   */
+  public function testDrupalMovingUploadedFileError() {
+    // Create a directory and make it not writable.
+    $test_directory = 'test_drupal_move_uploaded_file_fail';
+    drupal_mkdir('temporary://' . $test_directory, 0000);
+    $this->assertTrue(is_dir('temporary://' . $test_directory));
+
+    $edit = [
+      'file_subdir' => $test_directory,
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+    ];
+
+    \Drupal::state()->set('file_test.disable_error_collection', TRUE);
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('File upload error. Could not move uploaded file.'), 'Found the failure message.');
+    $this->assertRaw(t('Epic upload FAIL!'), 'Found the failure message.');
+
+    // Uploading failed. Now check the log.
+    $this->drupalGet('admin/reports/dblog');
+    $this->assertResponse(200);
+    $this->assertRaw(t('Upload error. Could not move uploaded file @file to destination @destination.', [
+      '@file' => $this->image->getFilename(),
+      '@destination' => 'temporary://' . $test_directory . '/' . $this->image->getFilename()
+    ]), 'Found upload error log entry.');
+  }
+
+  /**
+   * Tests that form validation does not change error messages.
+   */
+  public function testErrorMessagesAreNotChanged() {
+    $error = 'An error message set before file_save_upload_from_form()';
+
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'error_message' => $error,
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Ensure the expected error message is present and the counts before and
+    // after calling file_save_upload_from_form() are correct.
+    $this->assertText($error);
+    $this->assertRaw('Number of error messages before file_save_upload_from_form(): 1');
+    $this->assertRaw('Number of error messages after file_save_upload_from_form(): 1');
+
+    // Test that error messages are preserved when an error occurs.
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'error_message' => $error,
+      'extensions' => 'foo'
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('Epic upload FAIL!'), 'Found the failure message.');
+
+    // Ensure the expected error message is present and the counts before and
+    // after calling file_save_upload_from_form() are correct.
+    $this->assertText($error);
+    $this->assertRaw('Number of error messages before file_save_upload_from_form(): 1');
+    $this->assertRaw('Number of error messages after file_save_upload_from_form(): 1');
+
+    // Test a successful upload with no messages.
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('You WIN!'), 'Found the success message.');
+
+    // Ensure the error message is not present and the counts before and after
+    // calling file_save_upload_from_form() are correct.
+    $this->assertNoText($error);
+    $this->assertRaw('Number of error messages before file_save_upload_from_form(): 0');
+    $this->assertRaw('Number of error messages after file_save_upload_from_form(): 0');
+  }
+
+  /**
+   * Tests that multiple validation errors are combined in one message.
+   */
+  public function testCombinedErrorMessages() {
+    $textfile = current($this->drupalGetTestFiles('text'));
+    $this->assertTrue(is_file($textfile->uri), 'The text file we are going to upload exists.');
+
+    $edit = [
+      'files[file_test_upload][]' => [
+        drupal_realpath($this->phpfile->uri),
+        drupal_realpath($textfile->uri),
+      ],
+      'allow_all_extensions' => FALSE,
+      'is_image_file' => TRUE,
+      'extensions' => 'jpeg',
+    ];
+
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('Epic upload FAIL!'), 'Found the failure message.');
+
+    // Search for combined error message followed by a formatted list of messages.
+    $this->assertRaw(t('One or more files could not be uploaded.') . '<div class="item-list">', 'Error message contains combined list of validation errors.');
+  }
+
+  /**
+   * Tests highlighting of file upload field when it has an error.
+   */
+  public function testUploadFieldIsHighlighted() {
+    $this->assertEqual(0, count($this->cssSelect('input[name="files[file_test_upload][]"].error')), 'Successful file upload has no error.');
+
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'extensions' => 'foo'
+    ];
+    $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
+    $this->assertResponse(200, 'Received a 200 response for posted test file.');
+    $this->assertRaw(t('Epic upload FAIL!'), 'Found the failure message.');
+    $this->assertEqual(1, count($this->cssSelect('input[name="files[file_test_upload][]"].error')), 'File upload field has error.');
+  }
+
+}
diff --git a/core/modules/file/tests/file_test/file_test.routing.yml b/core/modules/file/tests/file_test/file_test.routing.yml
index cdbf08e..1c5a763 100644
--- a/core/modules/file/tests/file_test/file_test.routing.yml
+++ b/core/modules/file/tests/file_test/file_test.routing.yml
@@ -4,3 +4,9 @@ file.test:
     _form: 'Drupal\file_test\Form\FileTestForm'
   requirements:
     _access: 'TRUE'
+file.save_upload_from_form_test:
+  path: '/file-test/save_upload_from_form_test'
+  defaults:
+    _form: 'Drupal\file_test\Form\FileTestSaveUploadFromForm'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/file/tests/file_test/src/Form/FileTestSaveUploadFromForm.php b/core/modules/file/tests/file_test/src/Form/FileTestSaveUploadFromForm.php
new file mode 100644
index 0000000..0edf5a2
--- /dev/null
+++ b/core/modules/file/tests/file_test/src/Form/FileTestSaveUploadFromForm.php
@@ -0,0 +1,169 @@
+<?php
+
+namespace Drupal\file_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\State\StateInterface;
+use Drupal\file\FileFormTrait;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * File test form class.
+ */
+class FileTestSaveUploadFromForm extends FormBase {
+  use FileFormTrait;
+
+  /**
+   * Stores the state storage service.
+   *
+   * @var \Drupal\Core\State\StateInterface
+   */
+  protected $state;
+
+  /**
+   * Constructs a FileTestSaveUploadFromForm object.
+   *
+   * @param \Drupal\Core\State\StateInterface $state
+   *   The state key value store.
+   */
+  public function __construct(StateInterface $state) {
+    $this->state = $state;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('state')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return '_file_test_save_upload_from_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['file_test_upload'] = [
+      '#type' => 'file',
+      '#multiple' => TRUE,
+      '#title' => $this->t('Upload a file'),
+    ];
+    $form['file_test_replace'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Replace existing image'),
+      '#options' => [
+        FILE_EXISTS_RENAME => $this->t('Appends number until name is unique'),
+        FILE_EXISTS_REPLACE => $this->t('Replace the existing file'),
+        FILE_EXISTS_ERROR => $this->t('Fail with an error'),
+      ],
+      '#default_value' => FILE_EXISTS_RENAME,
+    ];
+    $form['file_subdir'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Subdirectory for test file'),
+      '#default_value' => '',
+    ];
+
+    $form['extensions'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Allowed extensions.'),
+      '#default_value' => '',
+    ];
+
+    $form['allow_all_extensions'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Allow all extensions?'),
+      '#default_value' => FALSE,
+    ];
+
+    $form['is_image_file'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Is this an image file?'),
+      '#default_value' => TRUE,
+    ];
+
+    $form['error_message'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Custom error message.'),
+      '#default_value' => '',
+    ];
+
+    $form['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Submit'),
+    ];
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    // Process the upload and perform validation. Note: we're using the
+    // form value for the $replace parameter.
+    if (!$form_state->isValueEmpty('file_subdir')) {
+      $destination = 'temporary://' . $form_state->getValue('file_subdir');
+      file_prepare_directory($destination, FILE_CREATE_DIRECTORY);
+    }
+    else {
+      $destination = FALSE;
+    }
+
+    // Preset custom error message if requested.
+    if ($form_state->getValue('error_message')) {
+      drupal_set_message($form_state->getValue('error_message'), 'error');
+    }
+
+    // Setup validators.
+    $validators = [];
+    if ($form_state->getValue('is_image_file')) {
+      $validators['file_validate_is_image'] = [];
+    }
+
+    if ($form_state->getValue('allow_all_extensions')) {
+      $validators['file_validate_extensions'] = [];
+    }
+    elseif (!$form_state->isValueEmpty('extensions')) {
+      $validators['file_validate_extensions'] = [$form_state->getValue('extensions')];
+    }
+
+    // The test for drupal_move_uploaded_file() triggering a warning is
+    // unavoidable. We're interested in what happens afterwards in
+    // file_save_upload_from_form().
+    if ($this->state->get('file_test.disable_error_collection')) {
+      define('SIMPLETEST_COLLECT_ERRORS', FALSE);
+    }
+
+    $form['file_test_upload']['#upload_validators'] = $validators;
+    $form['file_test_upload']['#upload_location'] = $destination;
+
+    drupal_set_message($this->t('Number of error messages before file_save_upload_from_form(): @count.', ['@count' => count(drupal_get_messages('error', FALSE))]));
+    $file = $this->fileSaveUpload($form['file_test_upload'], $form_state, 0, $form_state->getValue('file_test_replace'));
+    drupal_set_message($this->t('Number of error messages after file_save_upload_from_form(): @count.', ['@count' => count(drupal_get_messages('error', FALSE))]));
+
+    if ($file) {
+      $form_state->setValue('file_test_upload', $file);
+      drupal_set_message($this->t('File @filepath was uploaded.', ['@filepath' => $file->getFileUri()]));
+      drupal_set_message($this->t('File name is @filename.', ['@filename' => $file->getFilename()]));
+      drupal_set_message($this->t('File MIME type is @mimetype.', ['@mimetype' => $file->getMimeType()]));
+      drupal_set_message($this->t('You WIN!'));
+    }
+    elseif ($file === FALSE) {
+      drupal_set_message($this->t('Epic upload FAIL!'), 'error');
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) { }
+
+}
diff --git a/core/modules/locale/src/Form/ImportForm.php b/core/modules/locale/src/Form/ImportForm.php
index 0ea7180..f344873 100644
--- a/core/modules/locale/src/Form/ImportForm.php
+++ b/core/modules/locale/src/Form/ImportForm.php
@@ -5,6 +5,7 @@
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\file\FileFormTrait;
 use Drupal\language\ConfigurableLanguageManagerInterface;
 use Drupal\language\Entity\ConfigurableLanguage;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -13,6 +14,7 @@
  * Form constructor for the translation import screen.
  */
 class ImportForm extends FormBase {
+  use FileFormTrait;
 
   /**
    * Uploaded file entity.
@@ -108,6 +110,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       ),
       '#size' => 50,
       '#upload_validators' => $validators,
+      '#upload_location' => 'translations://',
       '#attributes' => array('class' => array('file-import-input')),
     );
     $form['langcode'] = array(
@@ -154,7 +157,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
    * {@inheritdoc}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
-    $this->file = file_save_upload('file', $form['file']['#upload_validators'], 'translations://', 0);
+    $this->file = $this->fileSaveUpload($form['file'], $form_state, 0);
 
     // Ensure we have the file uploaded.
     if (!$this->file) {
diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index 37b5ea1..43cbc8d 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\StreamWrapper\PublicStream;
+use Drupal\file\FileFormTrait;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
@@ -212,7 +213,10 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
         '#type' => 'file',
         '#title' => t('Upload logo image'),
         '#maxlength' => 40,
-        '#description' => t("If you don't have direct file access to the server, use this field to upload your logo.")
+        '#description' => t("If you don't have direct file access to the server, use this field to upload your logo."),
+        '#upload_validators' => array(
+          'file_validate_is_image' => array(),
+        ),
       );
     }
 
@@ -252,7 +256,12 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
       $form['favicon']['settings']['favicon_upload'] = array(
         '#type' => 'file',
         '#title' => t('Upload favicon image'),
-        '#description' => t("If you don't have direct file access to the server, use this field to upload your shortcut icon.")
+        '#description' => t("If you don't have direct file access to the server, use this field to upload your shortcut icon."),
+        '#upload_validators' => array(
+          'file_validate_extensions' => array(
+            'ico png gif jpg jpeg apng svg',
+          ),
+        ),
       );
     }
 
@@ -355,37 +364,18 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     parent::validateForm($form, $form_state);
 
     if ($this->moduleHandler->moduleExists('file')) {
-      // Handle file uploads.
-      $validators = array('file_validate_is_image' => array());
-
       // Check for a new uploaded logo.
-      $file = file_save_upload('logo_upload', $validators, FALSE, 0);
-      if (isset($file)) {
-        // File upload was attempted.
-        if ($file) {
-          // Put the temporary file in form_values so we can save it on submit.
-          $form_state->setValue('logo_upload', $file);
-        }
-        else {
-          // File upload failed.
-          $form_state->setErrorByName('logo_upload', $this->t('The logo could not be uploaded.'));
-        }
+      $file = FileFormTrait::fileSaveUpload($form['logo']['settings']['logo_upload'], $form_state, 0);
+      if ($file) {
+        // Put the temporary file in form_values so we can save it on submit.
+        $form_state->setValue('logo_upload', $file);
       }
 
-      $validators = array('file_validate_extensions' => array('ico png gif jpg jpeg apng svg'));
-
       // Check for a new uploaded favicon.
-      $file = file_save_upload('favicon_upload', $validators, FALSE, 0);
-      if (isset($file)) {
-        // File upload was attempted.
-        if ($file) {
-          // Put the temporary file in form_values so we can save it on submit.
-          $form_state->setValue('favicon_upload', $file);
-        }
-        else {
-          // File upload failed.
-          $form_state->setErrorByName('favicon_upload', $this->t('The favicon could not be uploaded.'));
-        }
+      $file = FileFormTrait::fileSaveUpload($form['favicon']['settings']['favicon_upload'], $form_state, 0);
+      if ($file) {
+        // Put the temporary file in form_values so we can save it on submit.
+        $form_state->setValue('favicon_upload', $file);
       }
 
       // When intending to use the default logo, unset the logo_path.
