Ticket #39759: 39759.5.patch

src/wp-includes/theme-compat/embed-content.php

@@ -22 +22 @@
                         $thumbnail_id = get_the_ID();
                 }
 
+                /**
+                 * Filters the thumbnail image id for use in the embed template.
+                 *
+                 * @since 4.8.0
+                 *
+                 * @param int $thumbnail_id Attachment ID.
+                 */
+                $thumbnail_id = apply_filters( 'embed_thumbnail_id', $thumbnail_id );
+
                 if ( $thumbnail_id ) {
                         $aspect_ratio = 1;
                         $measurements = array( 1, 1 );