When setting the poster image for a video shortcode, set that image as the featured image for that attachment (if found) in the background. This AJAX functionality could be used for audio as well.

Introduces `attachment_url_to_postid()` to attempt to turn URLs into post IDs.

Fixes #27891.


git-svn-id: https://develop.svn.wordpress.org/trunk@29029 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/media.php

@@ -2642,6 +2642,11 @@ function edit_form_image_editor( $post ) {
 			$attr['height'] = $h;
 		}
 
+		$thumb_id = get_post_thumbnail_id( $attachment_id );
+		if ( ! empty( $thumb_id ) ) {
+			$attr['poster'] = wp_get_attachment_url( $thumb_id );
+		}
+
 		echo wp_video_shortcode( $attr );
 
 	endif; ?>