diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 8262e39..d331147 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -701,21 +701,19 @@ function file_cron() {
  *   there was an error. Function returns NULL if no file was uploaded.
  */
 function file_save_upload_from_form($element, FormStateInterface $form_state, $delta = NULL, $replace = FILE_EXISTS_RENAME) {
-  $destination = isset($element['#upload_location']) ? $element['#upload_location'] : FALSE;
-  $validators = isset($element['#upload_validators']) ? $element['#upload_validators'] : array();
+  $errors_before = drupal_get_messages('error', TRUE);
+  $upload_location = isset($element['#upload_location']) ? $element['#upload_location'] : FALSE;
   $upload_name = implode('_', $element['#parents']);
-  $errors_before = drupal_get_messages('error');
+  $upload_validators = isset($element['#upload_validators']) ? $element['#upload_validators'] : array();
 
-  $files = file_save_upload($upload_name, $validators, $destination, $delta, $replace);
+  $result = file_save_upload($upload_name, $upload_validators, $upload_location, $delta, $replace);
 
-  // Get all possible new errors that are generated while trying to save the
-  // upload.
-  $new_errors = array_diff(drupal_get_messages('error'), $errors_before);
+  // Get new errors that are generated while trying to save the upload.
+  $errors_new = drupal_get_messages('error', TRUE);
+  if (!empty($errors_new['error'])) {
+    $errors_new = $errors_new['error'];
 
-  if (!empty($new_errors['error'])) {
-    $new_errors = $new_errors['error'];
-
-    if (count($new_errors) > 1) {
+    if (count($errors_new) > 1) {
       // Render multiple errors into a single message.
       $render_array = array(
         'error' => array(
@@ -723,33 +721,31 @@ function file_save_upload_from_form($element, FormStateInterface $form_state, $d
         ),
         'item_list' => array(
           '#theme' => 'item_list',
-          '#items' => $new_errors,
+          '#items' => $errors_new,
         ),
       );
       $error_message = \Drupal::service('renderer')->renderPlain($render_array);
     }
     else {
-      $error_message = reset($new_errors);
+      $error_message = reset($errors_new);
     }
 
     $form_state->setError($element, $error_message);
+  }
 
-    // Empty the old queue and put the errors back as before.
-    drupal_get_messages('error', TRUE);
-    foreach ($errors_before as $error) {
+  // Put the errors back as before.
+  if (!empty($errors_before['error'])) {
+    foreach ($errors_before['error'] as $error) {
       drupal_set_message($error, 'error');
     }
   }
-  return $files;
+
+  return $result;
 }
 
 /**
  * Saves file uploads to a new location.
  *
- * @deprecated in Drupal 8.3.0, intended to be removed in Drupal 9.0.0.
- * This function should not be used during form validation, use
- * file_save_upload_from_form() instead.
- *
  * 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.
@@ -784,6 +780,12 @@ function file_save_upload_from_form($element, FormStateInterface $form_state, $d
  *   An array of file entities or a single file entity if $delta != NULL. Each
  *   array element contains the file entity if the upload succeeded or FALSE if
  *   there was an error. Function returns NULL if no file was uploaded.
+ *
+ * @deprecated in Drupal 8.3.x, will be removed before Drupal 9.0.0.
+ *   This function should not be used for form validation, use
+ *   file_save_upload_from_form() instead.
+ *
+ * @see file_save_upload_from_form()
  */
 function file_save_upload($form_field_name, $validators = array(), $destination = FALSE, $delta = NULL, $replace = FILE_EXISTS_RENAME) {
   $user = \Drupal::currentUser();
diff --git a/core/modules/file/src/Tests/SaveUploadFormTest.php b/core/modules/file/src/Tests/SaveUploadFormTest.php
index 5d7f89f..90e8217 100644
--- a/core/modules/file/src/Tests/SaveUploadFormTest.php
+++ b/core/modules/file/src/Tests/SaveUploadFormTest.php
@@ -63,7 +63,7 @@ protected function setUp() {
     // Upload with replace to guarantee there's something there.
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri()),
+      '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.');
@@ -91,7 +91,7 @@ public function testNormal() {
 
     // Upload a second file.
     $image2 = current($this->drupalGetTestFiles('image'));
-    $edit = array('files[file_test_upload]' => drupal_realpath($image2->uri));
+    $edit = array('files[file_test_upload][]' => drupal_realpath($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!'));
@@ -115,7 +115,7 @@ public function testNormal() {
     $image3_realpath = drupal_realpath($image3->uri);
     $dir = $this->randomMachineName();
     $edit = array(
-      'files[file_test_upload]' => $image3_realpath,
+      'files[file_test_upload][]' => $image3_realpath,
       'file_subdir' => $dir,
     );
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
@@ -135,7 +135,7 @@ public function testHandleExtension() {
     $extensions = 'foo';
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
       'extensions' => $extensions,
     );
 
@@ -155,7 +155,7 @@ public function testHandleExtension() {
     // Now tell file_save_upload() to allow the extension of our test image.
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
       'extensions' => $extensions,
     );
 
@@ -173,7 +173,7 @@ public function testHandleExtension() {
     // Now tell file_save_upload() to allow any extension.
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri()),
+      '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'));
@@ -194,7 +194,7 @@ public function testHandleDangerousFile() {
     // safety. Also check to make sure its MIME type was changed.
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->phpfile->uri),
+      'files[file_test_upload][]' => drupal_realpath($this->phpfile->uri),
       'is_image_file' => FALSE,
       'extensions' => 'php',
     );
@@ -241,7 +241,7 @@ public function testHandleFileMunge() {
 
     $extensions = $this->imageExtension;
     $edit = array(
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
       'extensions' => $extensions,
     );
 
@@ -263,7 +263,7 @@ public function testHandleFileMunge() {
     file_test_reset();
 
     $edit = array(
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
       'allow_all_extensions' => TRUE,
     );
 
@@ -283,7 +283,7 @@ public function testHandleFileMunge() {
   public function testExistingRename() {
     $edit = array(
       'file_test_replace' => FILE_EXISTS_RENAME,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri())
+      '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.');
@@ -299,7 +299,7 @@ public function testExistingRename() {
   public function testExistingReplace() {
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri())
+      '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.');
@@ -315,7 +315,7 @@ public function testExistingReplace() {
   public function testExistingError() {
     $edit = array(
       'file_test_replace' => FILE_EXISTS_ERROR,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri())
+      '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.');
@@ -344,7 +344,7 @@ public function testDrupalMovingUploadedFileError() {
 
     $edit = array(
       'file_subdir' => $test_directory,
-      'files[file_test_upload]' => drupal_realpath($this->image->getFileUri())
+      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
     );
 
     \Drupal::state()->set('file_test.disable_error_collection', TRUE);
@@ -362,4 +362,65 @@ public function testDrupalMovingUploadedFileError() {
     )), 'Found upload error log entry.');
   }
 
+  /**
+   * Tests form validation does not change error messages.
+   */
+  public function testErrorMessagesAreNotChanged() {
+    $error = $this->randomMachineName();
+
+    $edit = [
+      'files[file_test_upload][]' => drupal_realpath($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.');
+
+    $this->assertRaw(t('Error message @key before validation: @message', array('@key' => 0, '@message' => $error)), 'Custom error message is set before validation.');
+    $this->assertRaw(t('Number of error messages before validation: @count', array('@count' => 1)), 'No extra error messages are set before validation.');
+    $this->assertRaw(t('Error message @key after validation: @message', array('@key' => 0, '@message' => $error)), 'Custom error message is set after validation.');
+    $this->assertRaw(t('Number of error messages after validation: @count', array('@count' => 1)), 'No extra error messages are set after validation.');
+  }
+
+  /**
+   * Test 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 that file upload field with highlighted when there is 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/src/Form/FileTestSaveUploadFromForm.php b/core/modules/file/tests/file_test/src/Form/FileTestSaveUploadFromForm.php
index 8aa39b4..8500e39 100644
--- a/core/modules/file/tests/file_test/src/Form/FileTestSaveUploadFromForm.php
+++ b/core/modules/file/tests/file_test/src/Form/FileTestSaveUploadFromForm.php
@@ -23,6 +23,7 @@ public function getFormId() {
   public function buildForm(array $form, FormStateInterface $form_state) {
     $form['file_test_upload'] = array(
       '#type' => 'file',
+      '#multiple' => TRUE,
       '#title' => t('Upload a file'),
     );
     $form['file_test_replace'] = array(
@@ -59,6 +60,12 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#default_value' => TRUE,
     );
 
+    $form['error_message'] = array(
+      '#type' => 'textfield',
+      '#title' => t('Custom error message.'),
+      '#default_value' => '',
+    );
+
     $form['submit'] = array(
       '#type' => 'submit',
       '#value' => t('Submit'),
@@ -80,6 +87,11 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
       $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.
     $validators = array();
     if ($form_state->getValue('is_image_file')) {
@@ -103,7 +115,10 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     $form['file_test_upload']['#upload_validators'] = $validators;
     $form['file_test_upload']['#upload_location'] = $destination;
 
+    $messages_before = drupal_get_messages('error', FALSE) + array('error' => array());
     $file = file_save_upload_from_form($form['file_test_upload'], $form_state, 0, $form_state->getValue('file_test_replace'));
+    $messages_after = drupal_get_messages('error', FALSE) + array('error' => array());
+
     if ($file) {
       $form_state->setValue('file_test_upload', $file);
       drupal_set_message(t('File @filepath was uploaded.', array('@filepath' => $file->getFileUri())));
@@ -114,6 +129,16 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     elseif ($file === FALSE) {
       drupal_set_message(t('Epic upload FAIL!'), 'error');
     }
+
+    drupal_set_message(t('Number of error messages before validation: @count', array('@count' => count($messages_before['error']))));
+    foreach ($messages_before['error'] as $key => $message) {
+      drupal_set_message(t('Error message @key before validation: @message', array('@key' => $key, '@message' => $message)));
+    }
+
+    drupal_set_message(t('Number of error messages after validation: @count', array('@count' => count($messages_after['error']))));
+    foreach ($messages_after['error'] as $key => $message) {
+      drupal_set_message(t('Error message @key after validation: @message', array('@key' => $key, '@message' => $message)));
+    }
   }
 
   /**