diff --git a/core/modules/media/src/Annotation/MediaSource.php b/core/modules/media/src/Annotation/MediaSource.php
index 2440ebec53..a1ab2231c4 100644
--- a/core/modules/media/src/Annotation/MediaSource.php
+++ b/core/modules/media/src/Annotation/MediaSource.php
@@ -80,14 +80,19 @@ class MediaSource extends Plugin {
   public $thumbnail_uri_metadata_attribute = 'thumbnail_uri';
 
   /**
-   * The metadata attribute name to provide the thumbnail alt.
+   * (optional) The metadata attribute name to provide the thumbnail alt.
+   *
+   * "Thumbnail" will be used if the attribute name is not provided.
    *
    * @var string|null
    */
   public $thumbnail_alt_metadata_attribute;
 
   /**
-   * The metadata attribute name to provide the thumbnail title.
+   * (optional) The metadata attribute name to provide the thumbnail title.
+   *
+   * The name of the media entity will be used if the attribute name is not
+   * provided.
    *
    * @var string|null
    */
diff --git a/core/modules/media/src/Entity/Media.php b/core/modules/media/src/Entity/Media.php
index e6538a4706..f9c06093ac 100644
--- a/core/modules/media/src/Entity/Media.php
+++ b/core/modules/media/src/Entity/Media.php
@@ -139,7 +139,7 @@ public function getSource() {
    * Update the thumbnail for the media item.
    *
    * @param bool $from_queue
-   *   Specifies whether the thumbnail is being fetched from the queue.
+   *   Specifies whether the thumbnail update is triggered from the queue.
    *
    * @return \Drupal\media\MediaInterface
    *   The updated media item.
@@ -165,7 +165,7 @@ protected function updateThumbnail($from_queue = FALSE) {
       $this->thumbnail->target_id = $file->id();
     }
 
-    // Set the image alt.
+    // Set the thumbnail alt.
     $plugin_definition = $this->getSource()->getPluginDefinition();
     if (!empty($plugin_definition['thumbnail_alt_metadata_attribute'])) {
       $this->thumbnail->alt = $this->getSource()->getMetadata($this, $plugin_definition['thumbnail_alt_metadata_attribute']);
@@ -174,7 +174,7 @@ protected function updateThumbnail($from_queue = FALSE) {
       $this->thumbnail->alt = $this->t('Thumbnail');
     }
 
-    // Set the image title.
+    // Set the thumbnail title.
     if (!empty($plugin_definition['thumbnail_title_metadata_attribute'])) {
       $this->thumbnail->title = $this->getSource()->getMetadata($this, $plugin_definition['thumbnail_title_metadata_attribute']);
     }
diff --git a/core/modules/media/src/MediaSourceInterface.php b/core/modules/media/src/MediaSourceInterface.php
index 95774c8cb9..1036907507 100644
--- a/core/modules/media/src/MediaSourceInterface.php
+++ b/core/modules/media/src/MediaSourceInterface.php
@@ -62,6 +62,8 @@
  * @see \Drupal\media\MediaSourceBase
  * @see \Drupal\media\MediaSourceManager
  * @see \Drupal\media\MediaTypeInterface
+ * @see \Drupal\media\MediaSourceEntityConstraintsInterface
+ * @see \Drupal\media\MediaSourceFieldConstraintsInterface
  * @see plugin_api
  */
 interface MediaSourceInterface extends PluginInspectionInterface, ConfigurablePluginInterface, PluginFormInterface {
diff --git a/core/modules/media/src/Plugin/QueueWorker/ThumbnailDownloader.php b/core/modules/media/src/Plugin/QueueWorker/ThumbnailDownloader.php
index b79605c47a..d30a232da1 100644
--- a/core/modules/media/src/Plugin/QueueWorker/ThumbnailDownloader.php
+++ b/core/modules/media/src/Plugin/QueueWorker/ThumbnailDownloader.php
@@ -5,7 +5,6 @@
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\media\Entity\Media;
 use Drupal\Core\Queue\QueueWorkerBase;
-use Drupal\media\MediaThumbnailHandlerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
diff --git a/core/modules/media/tests/modules/media_test_source/src/Plugin/media/Source/Test.php b/core/modules/media/tests/modules/media_test_source/src/Plugin/media/Source/Test.php
index 2e6653559d..b93e482ded 100644
--- a/core/modules/media/tests/modules/media_test_source/src/Plugin/media/Source/Test.php
+++ b/core/modules/media/tests/modules/media_test_source/src/Plugin/media/Source/Test.php
@@ -27,7 +27,9 @@ public function getMetadataAttributes() {
       'attribute_1' => ['label' => $this->t('Attribute 1'), 'value' => 'Value 1'],
       'attribute_2' => ['label' => $this->t('Attribute 2'), 'value' => 'Value 1'],
     ]);
-    return array_map(function ($item) {return $item['label'];}, $attributes);
+    return array_map(function ($item) {
+      return $item['label'];
+    }, $attributes);
   }
 
   /**
diff --git a/core/modules/media/tests/src/Kernel/MediaSourceTest.php b/core/modules/media/tests/src/Kernel/MediaSourceTest.php
index f6b4a69514..365370987f 100644
--- a/core/modules/media/tests/src/Kernel/MediaSourceTest.php
+++ b/core/modules/media/tests/src/Kernel/MediaSourceTest.php
@@ -3,6 +3,8 @@
 namespace Drupal\Tests\media\Kernel;
 
 use Drupal\Core\Entity\EntityStorageException;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\media\Entity\Media;
 use Drupal\media\Entity\MediaType;
 
@@ -55,22 +57,18 @@ public function testDefaultName() {
   public function testMetadataMapping() {
     $field_name = 'field_to_map_to';
     $attribute_name = 'attribute_to_map';
-    $storage = $this->container->get('entity_type.manager')
-      ->getStorage('field_storage_config')
-      ->create([
-        'entity_type' => 'media',
-        'field_name' => $field_name,
-        'type' => 'string',
-      ]);
+    $storage = FieldStorageConfig::create([
+      'entity_type' => 'media',
+      'field_name' => $field_name,
+      'type' => 'string',
+    ]);
     $storage->save();
 
-    $this->container->get('entity_type.manager')
-      ->getStorage('field_config')
-      ->create([
-        'field_storage' => $storage,
-        'bundle' => $this->testMediaType->id(),
-        'label' => 'Field to map to',
-      ])->save();
+    FieldConfig::create([
+      'field_storage' => $storage,
+      'bundle' => $this->testMediaType->id(),
+      'label' => 'Field to map to',
+    ])->save();
 
     // Save the entity without defining the metadata mapping and check that the
     // field stays empty.
@@ -84,7 +82,7 @@ public function testMetadataMapping() {
 
     // Define mapping and make sure that the value was stored in the field.
     \Drupal::state()->set('media_source_test_attributes', [
-      $attribute_name => ['title' => 'Attribute to map', 'value' => 'Snowball']
+      $attribute_name => ['title' => 'Attribute to map', 'value' => 'Snowball'],
     ]);
     $this->testMediaType->setFieldMap([$attribute_name => $field_name])->save();
     $media = Media::create([
@@ -98,7 +96,7 @@ public function testMetadataMapping() {
     // Change the metadata attribute value and re-save the entity. Field value
     // should stay the same.
     \Drupal::state()->set('media_source_test_attributes', [
-      $attribute_name => ['title' => 'Attribute to map', 'value' => 'Pinkeye']
+      $attribute_name => ['title' => 'Attribute to map', 'value' => 'Pinkeye'],
     ]);
     $this->assertEquals('Pinkeye', $media->getSource()->getMetadata($media, $attribute_name), 'Value of the metadata attribute is correct.');
     $media->save();
@@ -110,10 +108,20 @@ public function testMetadataMapping() {
     $media->set('field_media_test', 'some_new_value');
     $media->save();
     $this->assertEquals('Pinkeye', $media->get($field_name)->value, 'Metadata attribute was correctly mapped to the field.');
+
+    // Remove the value of the mapped field and make sure that it is re-mapped
+    // on save.
+    \Drupal::state()->set('media_source_test_attributes', [
+      $attribute_name => ['title' => 'Attribute to map', 'value' => 'Snowball'],
+    ]);
+    $media->{$field_name}->value = NULL;
+    $this->assertEquals('Snowball', $media->getSource()->getMetadata($media, $attribute_name), 'Value of the metadata attribute is correct.');
+    $media->save();
+    $this->assertEquals('Snowball', $media->get($field_name)->value, 'Metadata attribute was correctly mapped to the field.');
   }
 
   /**
-   * Tests the default thumbnail functionality.
+   * Tests the thumbnail functionality.
    */
   public function testThumbnail() {
     file_put_contents('public://thumbnail1.jpg', '');
@@ -143,15 +151,19 @@ public function testThumbnail() {
     $this->assertEquals('public://thumbnail2.jpg', $media->getSource()->getMetadata($media, 'thumbnail_uri'), 'Value of the thumbnail metadata attribute is correct.');
     $media->save();
     $this->assertEquals('public://thumbnail1.jpg', $media->thumbnail->entity->getFileUri(), 'Thumbnail was preserved.');
+    $this->assertEquals('Mr. Jones', $media->thumbnail->title, 'Title text was set on the thumbnail.');
+    $this->assertEquals('Thumbnail', $media->thumbnail->alt, 'Alt text was set on the thumbnail.');
 
     // Remove the thumbnail and make sure that it is auto-updated on save.
     $media->thumbnail->target_id = NULL;
     $this->assertEquals('public://thumbnail2.jpg', $media->getSource()->getMetadata($media, 'thumbnail_uri'), 'Value of the thumbnail metadata attribute is correct.');
     $media->save();
     $this->assertEquals('public://thumbnail2.jpg', $media->thumbnail->entity->getFileUri(), 'New thumbnail was added to the media entity.');
+    $this->assertEquals('Mr. Jones', $media->thumbnail->title, 'Title text was set on the thumbnail.');
+    $this->assertEquals('Thumbnail', $media->thumbnail->alt, 'Alt text was set on the thumbnail.');
 
-    // Change the metadata attribute again, change the source field value and
-    // make sure that the thumbnail updates too.
+    // Change the metadata attribute again, change the source field value too
+    // and make sure that the thumbnail updates.
     \Drupal::state()->set('media_source_test_attributes', [
       'thumbnail_uri' => ['title' => 'Thumbnail', 'value' => 'public://thumbnail1.jpg'],
     ]);
@@ -159,6 +171,8 @@ public function testThumbnail() {
     $this->assertEquals('public://thumbnail1.jpg', $media->getSource()->getMetadata($media, 'thumbnail_uri'), 'Value of the thumbnail metadata attribute is correct.');
     $media->save();
     $this->assertEquals('public://thumbnail1.jpg', $media->thumbnail->entity->getFileUri(), 'New thumbnail was added to the media entity.');
+    $this->assertEquals('Mr. Jones', $media->thumbnail->title, 'Title text was set on the thumbnail.');
+    $this->assertEquals('Thumbnail', $media->thumbnail->alt, 'Alt text was set on the thumbnail.');
 
     // Change the thumbnail metadata attribute and make sure that the thumbnail
     // is set correctly.
@@ -176,6 +190,8 @@ public function testThumbnail() {
     $this->assertEquals('public://thumbnail2.jpg', $media->getSource()->getMetadata($media, 'alternative_thumbnail_uri'), 'Value of the thumbnail metadata attribute is correct.');
     $media->save();
     $this->assertEquals('public://thumbnail2.jpg', $media->thumbnail->entity->getFileUri(), 'Correct metadata attribute was used for the thumbnail.');
+    $this->assertEquals('Mr. Jones', $media->thumbnail->title, 'Title text was set on the thumbnail.');
+    $this->assertEquals('Thumbnail', $media->thumbnail->alt, 'Alt text was set on the thumbnail.');
 
     // Enable queued thumbnails and make sure that the entity gets the default
     // thumbnail initially.
@@ -192,6 +208,8 @@ public function testThumbnail() {
     $this->assertEquals('public://thumbnail1.jpg', $media->getSource()->getMetadata($media, 'thumbnail_uri'), 'Value of the metadata attribute is correct.');
     $media->save();
     $this->assertEquals('public://media-icons/generic/generic.png', $media->thumbnail->entity->getFileUri(), 'Default thumbnail was set initially.');
+    $this->assertEquals('Mr. Jones', $media->thumbnail->title, 'Title text was set on the thumbnail.');
+    $this->assertEquals('Thumbnail', $media->thumbnail->alt, 'Alt text was set on the thumbnail.');
 
     // Process the queue item and make sure that the thumbnail was updated too.
     $queue_name = 'media_entity_thumbnail';
@@ -209,6 +227,8 @@ public function testThumbnail() {
 
     $media = Media::load($media->id());
     $this->assertEquals('public://thumbnail1.jpg', $media->thumbnail->entity->getFileUri(), 'Thumbnail was updated by the queue.');
+    $this->assertEquals('Mr. Jones', $media->thumbnail->title, 'Title text was set on the thumbnail.');
+    $this->assertEquals('Thumbnail', $media->thumbnail->alt, 'Alt text was set on the thumbnail.');
 
     // Set alt and title metadata attributes and make sure they are used for the
     // thumbnail.
@@ -240,8 +260,8 @@ public function testConstraints() {
       'MediaTestConstraint' => [],
     ]);
 
-    // Create media that uses source with constraints and make sure it can't
-    // be saved without validating them.
+    // Create media that uses source plugin with constraints and make sure it
+    // can't be saved without validating them.
     /** @var \Drupal\media\MediaInterface $media */
     $media = Media::create([
       'bundle' => $this->testConstraintsMediaType->id(),
@@ -342,17 +362,15 @@ public function testSourceFieldCreation() {
     $this->assertEquals('Test source', $field->label(), 'Correct label is used.');
     $this->assertEquals('test_type', $field->getTargetBundle(), 'Field is targeting correct bundle.');
 
-    // Fields should be automatically saved only when using a form.
+    // Fields should be automatically saved only when creating the media type
+    // using the media type creation form. Make sure that they are not saved
+    // when creating a media type programatically.
     // Drupal\Tests\media\FunctionalJavascript\MediaTypeCreationTest is testing
     // form part of the functionality.
     $type->save();
-    $storage = $this->container->get('entity_type.manager')
-      ->getStorage('field_storage_config')
-      ->load('media.field_media_test_1');
+    $storage = FieldStorageConfig::load('media.field_media_test_1');
     $this->assertNull($storage, 'Field storage was not saved.');
-    $field = $this->container->get('entity_type.manager')
-      ->getStorage('field_config')
-      ->load('media.test_type.field_media_test_1');
+    $field = FieldConfig::load('media.test_type.field_media_test_1');
     $this->assertNull($field, 'Field storage was not saved.');
 
     // Test the plugin with a different default source field type.