diff -u b/core/modules/media/src/Plugin/media/Source/File.php b/core/modules/media/src/Plugin/media/Source/File.php
--- b/core/modules/media/src/Plugin/media/Source/File.php
+++ b/core/modules/media/src/Plugin/media/Source/File.php
@@ -65,19 +65,18 @@
    * Gets the thumbnail image URI based on a file entity.
    *
    * @param \Drupal\file\FileInterface $file
-   *   The file entity.
+   *   A file entity.
    *
    * @return string
    *   File URI of the thumbnail image.
    */
   protected function getThumbnail(FileInterface $file) {
     $icon_base = $this->configFactory->get('media.settings')->get('icon_base');
-    $thumbnail = FALSE;
 
-    // We try to magically use the most specific icon present in the $icon_base
-    // directory, based on the MIME information. For instance, if an icon file
-    // named "pdf.png" is present, it will be used if the file matches this
-    // MIME type.
+    // We try to automatically use the most specific icon present in the
+    // $icon_base directory, based on the MIME type. For instance, if an
+    // icon file named "pdf.png" is present, it will be used if the file
+    // matches this MIME type.
     $mimetype = $file->getMimeType();
     $mimetype = explode('/', $mimetype);
 
diff -u b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceFileTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceFileTest.php
--- b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceFileTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceFileTest.php
@@ -12,10 +12,10 @@
 class MediaSourceFileTest extends MediaSourceTestBase {
 
   /**
-   * Just dummy test to check generic methods.
+   * Tests the file media source.
    */
   public function testMediaFileSource() {
-    $type_name = 'test_media_file_type';
+    $type_id = 'test_media_file_type';
     $source_field_id = 'field_media_file';
     $provided_fields = [
       'mime',
@@ -27,36 +27,40 @@
     $assert_session = $this->assertSession();
 
     // Create file media source.
-    $this->createMediaTypeTest($type_name, 'file', $provided_fields);
+    $this->createMediaTypeTest($type_id, 'file', $provided_fields);
 
-    // Create a supported and a non-supported field.
+    // Create a custom field for the media type to store the mime metadata
+    // attribute.
     $fields = [
       'field_string_mime' => 'string',
+      'field_string_size' => 'string',
     ];
-    $this->createMediaFields($fields, $type_name);
+    $this->createMediaTypeFields($fields, $type_id);
 
-    // Hide the media name to test default name generation.
-    $this->hideMediaField('name', $type_name);
+    // Hide the name field widget to test default name generation.
+    $this->hideMediaTypeFieldWidget('name', $type_id);
 
-    $this->drupalGet("admin/structure/media/manage/{$type_name}");
+    $this->drupalGet("admin/structure/media/manage/{$type_id}");
     $page->selectFieldOption("field_map[mime]", 'field_string_mime');
+    $page->selectFieldOption("field_map[size]", 'field_string_size');
     $page->pressButton('Save');
 
     // Create a media item.
-    $this->drupalGet("media/add/{$type_name}");
+    $this->drupalGet("media/add/{$type_id}");
     $page->attachFileToField("files[{$source_field_id}_0]", \Drupal::root() . '/sites/README.txt');
     $assert_session->assertWaitOnAjaxRequest();
     $page->pressButton('Save and publish');
 
     $assert_session->addressEquals('media/1');
 
-    // Make sure the thumbnail is created.
+    // Make sure the thumbnail is displayed.
     $assert_session->elementAttributeContains('css', '.image-style-thumbnail', 'src', 'generic.png');
 
     // Load the media and check that all fields are properly populated.
     $media = Media::load(1);
     $this->assertEquals('README.txt', $media->label());
     $this->assertEquals('text/plain', $media->get('field_string_mime')->value);
+    $this->assertEquals(filesize(\Drupal::root() . '/sites/README.txt'), $media->get('field_string_size')->value);
   }
 
 }
diff -u b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php
--- b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php
@@ -13,16 +13,16 @@
 abstract class MediaSourceTestBase extends MediaJavascriptTestBase {
 
   /**
-   * Create storage and field instance, attached to a given media type.
+   * Creates storage and field instance, attached to a given media type.
    *
    * @param string $field_name
    *   The field name.
    * @param string $field_type
    *   The field storage type.
-   * @param string $type_name
-   *   The media type machine name.
+   * @param string $type_id
+   *   The media type config entity ID.
    */
-  protected function createMediaField($field_name, $field_type, $type_name) {
+  protected function createMediaTypeField($field_name, $field_type, $type_id) {
     $storage = FieldStorageConfig::create([
       'field_name' => $field_name,
       'entity_type' => 'media',
@@ -30,24 +30,23 @@
     ]);
     $storage->save();
 
-    $config = FieldConfig::create([
+    FieldConfig::create([
       'field_storage' => $storage,
-      'bundle' => $type_name,
-    ]);
-    $config->save();
+      'bundle' => $type_id,
+    ])->save();
 
-    // Make the field visible in the form display.
+    // Make the field widget visible in the form display.
     $component = \Drupal::service('plugin.manager.field.widget')
       ->prepareConfiguration($field_type, []);
 
     /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $entity_form_display */
     $entity_form_display = \Drupal::entityTypeManager()
       ->getStorage('entity_form_display')
-      ->load('media.' . $type_name . '.default');
+      ->load('media.' . $type_id . '.default');
     if (!$entity_form_display) {
       $entity_form_display = EntityFormDisplay::create(array(
         'targetEntityType' => 'media',
-        'bundle' => $type_name,
+        'bundle' => $type_id,
         'mode' => 'default',
         'status' => TRUE,
       ));
@@ -57,40 +56,40 @@
   }
 
   /**
-   * Helper to create a set of fields in a media type.
+   * Create a set of fields in a media type.
    *
    * @param array $fields
    *   An associative array where keys are field names and values field types.
-   * @param string $type_name
-   *   The type machine name.
+   * @param string $type_id
+   *   The media type config entity ID.
    */
-  protected function createMediaFields(array $fields, $type_name) {
+  protected function createMediaTypeFields(array $fields, $type_id) {
     foreach ($fields as $field_name => $field_type) {
-      $this->createMediaField($field_name, $field_type, $type_name);
+      $this->createMediaTypeField($field_name, $field_type, $type_id);
     }
   }
 
   /**
-   * Hide a component from the default form display config.
+   * Hides a widget in the default form display config.
    *
    * @param string $field_name
    *   The field name.
-   * @param string $type_name
-   *   The media type machine name.
+   * @param string $type_id
+   *   The media type config entity ID.
    */
-  protected function hideMediaField($field_name, $type_name) {
+  protected function hideMediaTypeFieldWidget($field_name, $type_id) {
     /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $entity_form_display */
     $entity_form_display = \Drupal::entityTypeManager()
       ->getStorage('entity_form_display')
-      ->load('media.' . $type_name . '.default');
+      ->load('media.' . $type_id . '.default');
     $entity_form_display->removeComponent($field_name)->save();
   }
 
   /**
-   * Helper to test a generic type (bundle) creation.
+   * Test generic media type creation.
    *
-   * @param string $type_name
-   *   The type machine name.
+   * @param string $type_id
+   *   The media type config entity ID.
    * @param string $source_id
    *   The media source ID.
    * @param array $provided_fields
@@ -99,14 +98,14 @@
    * @return \Drupal\media\MediaTypeInterface
    *   The type created.
    */
-  public function createMediaTypeTest($type_name, $source_id, array $provided_fields = []) {
+  public function createMediaTypeTest($type_id, $source_id, array $provided_fields = []) {
     $session = $this->getSession();
     $page = $session->getPage();
     $assert_session = $this->assertSession();
 
     $this->drupalGet('admin/structure/media/add');
-    $page->fillField('label', $type_name);
-    $this->assertJsCondition("jQuery('.machine-name-value').text() === '{$type_name}'");
+    $page->fillField('label', $type_id);
+    $this->assertJsCondition("jQuery('.machine-name-value').text() === '{$type_id}'");
 
     // Make sure the source is available.
     $this->assertSession()->fieldExists('Media source');
@@ -124,16 +123,16 @@
     // Save the page to create the type.
     $page->pressButton('Save');
     $assert_session->statusCodeEquals(200);
-    $assert_session->pageTextContains('The media type ' . $type_name . ' has been added.');
+    $assert_session->pageTextContains('The media type ' . $type_id . ' has been added.');
     $this->drupalGet('admin/structure/media');
-    $assert_session->pageTextContains($type_name);
+    $assert_session->pageTextContains($type_id);
 
     // Bundle definitions are statically cached in the context of the test, we
     // need to make sure we have updated information before proceeding with the
     // actions on the UI.
     \Drupal::service('entity_type.bundle.info')->clearCachedBundles();
 
-    return MediaType::load($type_name);
+    return MediaType::load($type_id);
   }
 
 }