Ticket #41692: rest-thumbnail.diff

src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

@@ -286 +286 @@
                 $response = parent::prepare_item_for_response( $post, $request );
                 $data = $response->get_data();
 
+                if ( wp_attachment_is( 'audio', $post ) || wp_attachment_is( 'video', $post ) ) {
+                        $data['featured_media'] = (int) get_post_thumbnail_id( $post->ID );
+                }
+
                 $data['description'] = array(
                         'raw'       => $post->post_content,
                         /** This filter is documented in wp-includes/post-template.php */

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -1824 +1824 @@
                         'attachment' => array(
                                 'title',
                                 'author',
+                                'thumbnail',
                                 'comments',
                                 'revisions',
                                 'custom-fields',