diff --git a/core/modules/media/src/FileGuesser.php b/core/modules/media/src/FileGuesser.php
index e90d4a852c..9a23fe5b35 100644
--- a/core/modules/media/src/FileGuesser.php
+++ b/core/modules/media/src/FileGuesser.php
@@ -8,11 +8,12 @@
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\file\FileInterface;
 use Drupal\file\Plugin\Field\FieldType\FileItem;
+use Drupal\media\FileGuesserInterface;
 
 /**
  * Discovers media types which support file uploads.
  */
-class FileGuesser {
+class FileGuesser implements FileGuesserInterface {
 
   use UseCacheBackendTrait;
 
@@ -45,16 +46,7 @@ public function __construct(EntityTypeManagerInterface $entity_type_manager, Cac
   }
 
   /**
-   * Returns the first media type ID that matches a file.
-   *
-   * @param \Drupal\file\FileInterface|int $file
-   *   A file entity or ID.
-   * @param string[] $limit
-   *   (optional) The media type IDs to choose from. If omitted, all media types
-   *   will be considered.
-   *
-   * @return string|bool
-   *   A media type ID, or FALSE if no media types matched.
+   * {@inheritdoc}
    */
   public function guessForFile($file, array $limit = []) {
     $media_types = $this->getTypesForFile($file, $limit);
@@ -62,16 +54,7 @@ public function guessForFile($file, array $limit = []) {
   }
 
   /**
-   * Returns the first media type ID that matches a file extension.
-   *
-   * @param string $extension
-   *   A file extension, without the leading period.
-   * @param string[] $limit
-   *   (optional) The media type IDs to choose from. If omitted, all media types
-   *   will be considered.
-   *
-   * @return string|bool
-   *   A media type ID, or FALSE if no media types matched.
+   * {@inheritdoc}
    */
   public function guessForExtension($extension, array $limit = []) {
     $media_types = $this->getTypesByExtension($extension, $limit);
@@ -79,19 +62,7 @@ public function guessForExtension($extension, array $limit = []) {
   }
 
   /**
-   * Returns all media types that match a file.
-   *
-   * @param \Drupal\file\FileInterface|int $file
-   *   A file entity or ID.
-   * @param string[] $limit
-   *   (optional) The media type IDs to choose from. If omitted, all media types
-   *   will be considered.
-   *
-   * @throws \InvalidArgumentException
-   *   If $file is not a file entity, or the ID of one.
-   *
-   * @return \Drupal\media\MediaTypeInterface[]
-   *   All matching media types.
+   * {@inheritdoc}
    */
   public function getTypesForFile($file, array $limit = []) {
     if (is_numeric($file)) {
@@ -109,16 +80,7 @@ public function getTypesForFile($file, array $limit = []) {
   }
 
   /**
-   * Returns all media types that match a file extension.
-   *
-   * @param string $extension
-   *   A file extension, without the leading period.
-   * @param string[] $limit
-   *   (optional) The media type IDs to choose from. If omitted, all media types
-   *   will be considered.
-   *
-   * @return \Drupal\media\MediaTypeInterface[]
-   *   All matching media types.
+   * {@inheritdoc}
    */
   public function getTypesByExtension($extension, array $limit = []) {
     $cache = $this->cacheGet($extension);
diff --git a/core/modules/media/src/FileGuesserInterface.php b/core/modules/media/src/FileGuesserInterface.php
new file mode 100644
index 0000000000..2cbddee69b
--- /dev/null
+++ b/core/modules/media/src/FileGuesserInterface.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Drupal\media;
+
+/**
+ * Discovers media types which support file uploads.
+ */
+interface FileGuesserInterface {
+  /**
+   * Returns the first media type ID that matches a file.
+   *
+   * @param \Drupal\file\FileInterface|int $file
+   *   A file entity or ID.
+   * @param string[] $limit
+   *   (optional) The media type IDs to choose from. If omitted, all media types
+   *   will be considered.
+   *
+   * @return string|bool
+   *   A media type ID, or FALSE if no media types matched.
+   */
+  public function guessForFile($file, array $limit = []);
+
+  /**
+   * Returns the first media type ID that matches a file extension.
+   *
+   * @param string $extension
+   *   A file extension, without the leading period.
+   * @param string[] $limit
+   *   (optional) The media type IDs to choose from. If omitted, all media types
+   *   will be considered.
+   *
+   * @return string|bool
+   *   A media type ID, or FALSE if no media types matched.
+   */
+  public function guessForExtension($extension, array $limit = []);
+
+  /**
+   * Returns all media types that match a file.
+   *
+   * @param \Drupal\file\FileInterface|int $file
+   *   A file entity or ID.
+   * @param string[] $limit
+   *   (optional) The media type IDs to choose from. If omitted, all media types
+   *   will be considered.
+   *
+   * @throws \InvalidArgumentException
+   *   If $file is not a file entity, or the ID of one.
+   *
+   * @return \Drupal\media\MediaTypeInterface[]
+   *   All matching media types.
+   */
+  public function getTypesForFile($file, array $limit = []);
+
+  /**
+   * Returns all media types that match a file extension.
+   *
+   * @param string $extension
+   *   A file extension, without the leading period.
+   * @param string[] $limit
+   *   (optional) The media type IDs to choose from. If omitted, all media types
+   *   will be considered.
+   *
+   * @return \Drupal\media\MediaTypeInterface[]
+   *   All matching media types.
+   */
+  public function getTypesByExtension($extension, array $limit = []);
+
+}
+
diff --git a/core/modules/media/src/Form/AddFileForm.php b/core/modules/media/src/Form/AddFileForm.php
index 41444026ab..83d0b72f80 100644
--- a/core/modules/media/src/Form/AddFileForm.php
+++ b/core/modules/media/src/Form/AddFileForm.php
@@ -7,15 +7,57 @@
 use Drupal\Core\Ajax\CloseModalDialogCommand;
 use Drupal\Core\Ajax\InvokeCommand;
 use Drupal\Core\Ajax\OpenDialogUrlCommand;
+use Drupal\Component\Datetime\TimeInterface;
+use Drupal\Core\Entity\EntityManagerInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Url;
 use Drupal\media\MediaForm;
+use Drupal\media\FileGuesserInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
 
 /**
  * Provides 'add_file' form handler for the Media entity.
  */
 class AddFileForm extends MediaForm {
 
+ /**
+  * The FileGesser service.
+  *
+  * @var Drupal\media\FileGuesserInterface
+  */
+  protected $fileGuessor;
+
+  /**
+   * Constructs a ContentEntityForm object.
+   *
+   * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
+   *   The entity manager.
+     @param Drupal\media\FileGuesserInterface
+       The file Guesser service.
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
+   *   (optional) The entity type bundle service.
+   * @param \Drupal\Component\Datetime\TimeInterface $time
+   *   (optional) The time service.
+   */
+  public function __construct(EntityManagerInterface $entity_manager, FileGuesserInterface $file_guesser, EntityTypeBundleInfoInterface $entity_type_bundle_info = NULL, TimeInterface $time = NULL) {
+    parent::__construct($entity_manager, $entity_type_bundle_info, $time);
+    $this->fileGuesser = $file_guesser;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity.manager'),
+      $container->get('media.type_guesser.file'),
+      $container->get('entity_type.bundle.info'),
+      $container->get('datetime.time')
+    );
+  }
+
   /**
    * AJAX callback associated with the 'media type' input selector.
    *
@@ -173,8 +215,7 @@ public function onSave(array &$form, FormStateInterface $form_state) {
       ];
       $next_file_id = array_shift($query['files']);
       $parameters['file'] = $next_file_id;
-      // TODO: Inject the media.type_guesser.file service.
-      $parameters['media_type'] = \Drupal::service('media.type_guesser.file')->guessForFile($next_file_id);
+      $parameters['media_type'] = $this->fileGuesser->guessForFile($next_file_id);
 
       $url = Url::fromRoute($route_match->getRouteName(), $parameters, [
         'query' => $query,