Index: src/wp-admin/admin-ajax.php
===================================================================
--- src/wp-admin/admin-ajax.php	(revision 29023)
+++ src/wp-admin/admin-ajax.php	(working copy)
@@ -60,7 +60,7 @@
 	'wp-remove-post-lock', 'dismiss-wp-pointer', 'upload-attachment', 'get-attachment',
 	'query-attachments', 'save-attachment', 'save-attachment-compat', 'send-link-to-editor',
 	'send-attachment-to-editor', 'save-attachment-order', 'heartbeat', 'get-revision-diffs',
-	'save-user-color-scheme', 'update-widget', 'query-themes', 'parse-embed'
+	'save-user-color-scheme', 'update-widget', 'query-themes', 'parse-embed', 'set-attachment-thumbnail'
 );
 
 // Register core Ajax calls.
Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 29023)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -1947,6 +1947,53 @@
 }
 
 /**
+ * Ajax handler for setting the featured image for an attachment.
+ *
+ * @since 4.0.0
+ */
+function wp_ajax_set_attachment_thumbnail() {
+	if ( empty( $_POST['urls'] ) || ! is_array( $_POST['urls'] ) ) {
+		wp_send_json_error();
+	}
+
+	$thumbnail_id = (int) $_POST['thumbnail_id'];
+	if ( empty( $thumbnail_id ) ) {
+		wp_send_json_error();
+	}	
+
+	$post_ids = array();
+	foreach ( $_POST['urls'] as $url ) {
+		$post_id = attachment_url_to_postid( $url );
+		if ( ! empty( $post_id ) ) {
+			$post_ids[] = $post_id;
+		}
+	}
+
+	if ( empty( $post_ids ) ) {
+		wp_send_json_error();
+	}
+
+	$success = 0;
+	foreach ( $post_ids as $post_id ) {
+		if ( ! current_user_can( 'edit_post', $post_id ) ) {
+			continue;
+		}
+
+		if ( set_post_thumbnail( $post_id, $thumbnail_id ) ) {
+			$success++;
+		}
+	}
+
+	if ( 0 === $success ) {
+		wp_send_json_error();
+	} else {
+		wp_send_json_success();
+	}
+
+	wp_send_json_error();
+}
+
+/**
  * Ajax handler for date formatting.
  *
  * @since 3.1.0
Index: src/wp-admin/includes/media.php
===================================================================
--- src/wp-admin/includes/media.php	(revision 29023)
+++ src/wp-admin/includes/media.php	(working copy)
@@ -2642,6 +2642,11 @@
 			$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; ?>
Index: src/wp-includes/js/media-audiovideo.js
===================================================================
--- src/wp-includes/js/media-audiovideo.js	(revision 29023)
+++ src/wp-includes/js/media-audiovideo.js	(working copy)
@@ -692,10 +692,23 @@
 
 		renderSelectPosterImageToolbar: function() {
 			this.setPrimaryButton( l10n.videoSelectPosterImageTitle, function( controller, state ) {
-				var attachment = state.get( 'selection' ).single();
+				var urls = [], attachment = state.get( 'selection' ).single();
 
 				controller.media.set( 'poster', attachment.get( 'url' ) );
 				state.trigger( 'set-poster-image', controller.media.toJSON() );
+
+				_.each( wp.media.view.settings.embedExts, function (ext) {
+					if ( controller.media.get( ext ) ) {
+						urls.push( controller.media.get( ext ) );
+					}
+				} );
+
+				wp.ajax.send( 'set-attachment-thumbnail', {
+					data : {
+						urls: urls,
+						thumbnail_id: attachment.get( 'id' )
+					}
+				} );
 			} );
 		},
 
Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 29023)
+++ src/wp-includes/media.php	(working copy)
@@ -3218,3 +3218,28 @@
 		}
 	}
 }
+
+/**
+ * Try to convert an attachment URL into a post ID.
+ *
+ * @since 4.0.0
+ *
+ * @global wpdb $wpdb WordPress database access abstraction object.
+ * @param string $url The URL to resolve.
+ * @return int The found post_id.
+ */
+function attachment_url_to_postid( $url ) {
+	global $wpdb;
+
+	$dir = wp_upload_dir();
+	$path = ltrim( $url, $dir['baseurl'] . '/' );
+
+	$sql = $wpdb->prepare(
+		"SELECT post_id FROM $wpdb->postmeta WHERE meta_key = '_wp_attached_file' AND meta_value = %s",
+		$path
+	);
+	$post_id = $wpdb->get_var( $sql );
+	if ( ! empty( $post_id ) ) {
+		return (int) $post_id;
+	}
+}
\ No newline at end of file