Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 58805)
+++ wp-includes/post.php	(working copy)
@@ -858,7 +858,9 @@
  *
  * @param int    $attachment_id Attachment ID.
  * @param string $file          File path for the attachment.
- * @return bool True on success, false on failure.
+ * @return int|bool Meta ID if the key didn't exist, true on success,
+ *                  false on failure or if the value passed to the function
+ *                  is the same as the one that is already in the database.
  */
 function update_attached_file( $attachment_id, $file ) {
 	if ( ! get_post( $attachment_id ) ) {