diff -u b/core/modules/media/src/MediaTypeResolverBase.php b/core/modules/media/src/MediaTypeResolverBase.php
--- b/core/modules/media/src/MediaTypeResolverBase.php
+++ b/core/modules/media/src/MediaTypeResolverBase.php
@@ -1,10 +1,13 @@
 <?php
 
-namespace Drupal\media_library;
+namespace Drupal\media;
 
 use Drupal\Core\Entity\EntityTypeManagerInterface;
-use Drupal\media\MediaTypeResolverInterface;
+use Drupal\Core\Form\FormStateInterface;
 
+/**
+ * Provides a base class for media type resolvers.
+ */
 abstract class MediaTypeResolverBase implements MediaTypeResolverInterface {
 
   /**
@@ -12,23 +15,45 @@
    *
    * @var \Drupal\Core\Entity\EntityStorageInterface
    */
-  protected $entityTypeManager;
+  protected $storage;
 
+  /**
+   * The media entity access control handler.
+   *
+   * @var \Drupal\Core\Entity\EntityAccessControlHandlerInterface
+   */
+  protected $accessHandler;
+
+  /**
+   * MediaTypeResolverBase constructor.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager service.
+   */
   public function __construct(EntityTypeManagerInterface $entity_type_manager) {
-    $this->entityTypeManager = $entity_type_manager;
+    $this->storage = $entity_type_manager->getStorage('media_type');
+    $this->accessHandler = $entity_type_manager->getAccessControlHandler('media');
   }
 
   /**
    * {@inheritdoc}
    */
   public function getAll(array $allowed_types = NULL) {
-    $media_types = $this->entityTypeManager->getStorage('media_type')->loadMultiple($allowed_types);
+    $media_types = $this->storage->loadMultiple($allowed_types);
 
     // Filter out any media types which the current user does not have
     // permission to create.
-    $access_handler = $this->entityTypeManager->getAccessControlHandler('media');
+    $allowed_types = array_keys($media_types);
+    $allowed_types = array_filter($allowed_types, [$this->accessHandler, 'createAccess']);
+
+    return array_intersect_key($media_types, array_flip($allowed_types));
+  }
 
-    return array_filter($media_types, [$access_handler, 'createAccess']);
+  /**
+   * {@inheritdoc}
+   */
+  public function alterFormElement(array &$element, array &$complete_form, FormStateInterface $form_state) {
+    // Don't do anything by default.
   }
 
 }
diff -u b/core/modules/media/src/MediaTypeResolverInterface.php b/core/modules/media/src/MediaTypeResolverInterface.php
--- b/core/modules/media/src/MediaTypeResolverInterface.php
+++ b/core/modules/media/src/MediaTypeResolverInterface.php
@@ -4,6 +4,21 @@
 
 use Drupal\Core\Form\FormStateInterface;
 
+/**
+ * Defines an interface for a media type resolver.
+ *
+ * Media type resolvers take arbitrary an input value and determine which media
+ * types, if any, can use that value in their source field. They can scan all
+ * available media types which match their internal critieria (for example, all
+ * media types using a specific source plugin), or a subset thereof, when
+ * making this determination.
+ *
+ * Media type resolvers can also alter form elements which will receive a value
+ * which may be used as the source value for a media item of indeterminate type.
+ * A good example of this is to attach generic validation criteria to a file
+ * upload element whose value (i.e., the uploaded file) will be used to create a
+ * new media item.
+ */
 interface MediaTypeResolverInterface {
 
   /**
diff -u b/core/modules/media_library/src/UploadMediaTypeResolver.php b/core/modules/media_library/src/UploadMediaTypeResolver.php
--- b/core/modules/media_library/src/UploadMediaTypeResolver.php
+++ b/core/modules/media_library/src/UploadMediaTypeResolver.php
@@ -8,22 +8,16 @@
 use Drupal\file\Plugin\Field\FieldType\FileFieldItemList;
 use Drupal\file\Plugin\Field\FieldType\FileItem;
 use Drupal\media\MediaTypeInterface;
+use Drupal\media\MediaTypeResolverBase;
 
+/**
+ * Media type resolver for uploaded files.
+ */
 class UploadMediaTypeResolver extends MediaTypeResolverBase {
 
   /**
    * {@inheritdoc}
    */
-  public function getAll(array $allowed_types = NULL) {
-    $media_types = parent::getAll($allowed_types);
-
-    // Filter out media types which do not use a file field as a source.
-    return array_filter($media_types, [$this, 'isFileSource']);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getAllByValue($value, array $allowed_types = NULL) {
     if ($value instanceof FileInterface) {
       $filter = function (MediaTypeInterface $media_type) use ($value) {
@@ -38,6 +32,16 @@
   /**
    * {@inheritdoc}
    */
+  public function getAll(array $allowed_types = NULL) {
+    $media_types = parent::getAll($allowed_types);
+
+    // Filter out media types which do not use a file field as a source.
+    return array_filter($media_types, [$this, 'isFileSource']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function alterFormElement(array &$element, array &$complete_form, FormStateInterface $form_state) {
     if ($element['#type'] === 'managed_file') {
       $allowed_types = isset($element['#allowed_types']) ? $element['#allowed_types'] : NULL;
@@ -46,6 +50,15 @@
     }
   }
 
+  /**
+   * Derives a set of upload validators from a set of media types.
+   *
+   * @param \Drupal\media\MediaTypeInterface[] $media_types
+   *   The media types, each of which uses a file or image field as its source.
+   *
+   * @return array
+   *   The upload validators, suitable for passing to file_validate().
+   */
   protected function mergeUploadValidators(array $media_types) {
     $max_size = 0;
     $extensions = [];
@@ -72,14 +85,36 @@
     ];
   }
 
+  /**
+   * Creates an empty file field item for a media type.
+   *
+   * @param \Drupal\media\MediaTypeInterface $media_type
+   *   A media type which uses a file or image field as its source.
+   *
+   * @return \Drupal\file\Plugin\Field\FieldType\FileItem
+   *   The empty field item.
+   */
   protected function getItem(MediaTypeInterface $media_type) {
     $field_definition = $media_type->getSource()->getSourceFieldDefinition($media_type);
     $data_definition = FieldItemDataDefinition::create($field_definition);
     return new FileItem($data_definition);
   }
 
+  /**
+   * Indicates if a media type uses a file or image field as its source.
+   *
+   * @param \Drupal\media\MediaTypeInterface $media_type
+   *   The media type.
+   *
+   * @return bool
+   *   TRUE if the media type's source field is a file or image field, otherwise
+   *   FALSE.
+   */
   protected function isFileSource(MediaTypeInterface $media_type) {
-    $class = $media_type->getSource()->getSourceFieldDefinition($media_type);
+    $class = $media_type->getSource()
+      ->getSourceFieldDefinition($media_type)
+      ->getClass();
+
     return is_a($class, FileFieldItemList::class, TRUE);
   }