diff --git a/core/includes/file.inc b/core/includes/file.inc
index b38cf14..79519ff 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -1152,32 +1152,6 @@ function file_upload_max_size() {
 }
 
 /**
- * Determines an Internet Media Type or MIME type from a filename.
- *
- * @param $uri
- *   A string containing the URI, path, or filename.
- * @param $mapping
- *   An optional map of extensions to their mimetypes, in the form:
- *    - 'mimetypes': a list of mimetypes, keyed by an identifier,
- *    - 'extensions': the mapping itself, an associative array in which the key
- *      is the extension (lowercase) and the value is the mimetype identifier.
- *
- * @return
- *   The internet media type registered for the extension or
- *   application/octet-stream for unknown extensions.
- *
- * @deprecated in Drupal 8.x-dev, will be removed before Drupal 8.0. Calls are
- *   passed on to a new file.mime_type.guesser service, and the $mapping
- *   parameter is ignored. Use
- *   \Drupal::service('file.mime_type.guesser')->guess($uri).
- *
- * @see \Drupal\Core\File\MimeType\ExtensionMimeTypeGuesser::guess()
- */
-function file_get_mimetype($uri, $mapping = NULL) {
-  return \Drupal::service('file.mime_type.guesser')->guess($uri);
-}
-
-/**
  * Sets the permissions on a file or directory.
  *
  * This function will use the file_chmod_directory and
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 1d78fb3..4a71b0f 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -758,7 +758,7 @@ function file_save_upload($form_field_name, $validators = array(), $destination
       'uri' => $file_info->getRealPath(),
       'filesize' => $file_info->getSize(),
     );
-    $values['filemime'] = file_get_mimetype($values['filename']);
+    $values['filemime'] = \Drupal::service('file.mime_type.guesser')->guess($values['uri']);
     $file = entity_create('file', $values);
 
     $extensions = '';
diff --git a/core/modules/file/src/Entity/File.php b/core/modules/file/src/Entity/File.php
index b21a182..7f89508 100644
--- a/core/modules/file/src/Entity/File.php
+++ b/core/modules/file/src/Entity/File.php
@@ -195,7 +195,7 @@ public static function preCreate(EntityStorageInterface $storage, array &$values
 
     // Automatically detect filemime if not set.
     if (!isset($values['filemime']) && isset($values['uri'])) {
-      $values['filemime'] = file_get_mimetype($values['uri']);
+      $values['filemime'] = \Drupal::service('file.mime_type.guesser')->guess($values['uri']);
     }
   }
 
diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index 44649d2..fa7a414 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Render\Element;
 use Drupal\Core\StreamWrapper\PublicStream;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Config\ConfigFactoryInterface;
@@ -30,17 +31,27 @@ class ThemeSettingsForm extends ConfigFormBase {
   protected $moduleHandler;
 
   /**
+   * The mime type guesser.
+   *
+   * @var \Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface
+   */
+  protected $mimeTypeGuesser;
+
+  /**
    * Constructs a ThemeSettingsForm object.
    *
    * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
    *   The factory for configuration objects.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface
    *   The module handler instance to use.
+   * @param \Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface $mime_type_guesser
+   *   The mime type guesser.
    */
-  public function __construct(ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler) {
+  public function __construct(ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler, MimeTypeGuesserInterface $mime_type_guesser) {
     parent::__construct($config_factory);
 
     $this->moduleHandler = $module_handler;
+    $this->mimeTypeGuesser = $mime_type_guesser;
   }
 
   /**
@@ -49,7 +60,8 @@ public function __construct(ConfigFactoryInterface $config_factory, ModuleHandle
   public static function create(ContainerInterface $container) {
     return new static(
       $container->get('config.factory'),
-      $container->get('module_handler')
+      $container->get('module_handler'),
+      $container->get('file.mime_type.guesser')
     );
   }
 
@@ -414,7 +426,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       }
 
       if (empty($values['default_favicon']) && !empty($values['favicon_path'])) {
-        $values['favicon_mimetype'] = file_get_mimetype($values['favicon_path']);
+        $values['favicon_mimetype'] = $this->mimeTypeGuesser->guess($values['favicon_path']);
       }
     }