Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 19953)
+++ wp-includes/functions.php	(working copy)
@@ -1557,7 +1557,7 @@
 		}
 	}
 
-	return $filename;
+	return apply_filters( 'wp_unique_filename', $filename, $ext, $dir, $unique_filename_callback );
 }
 
 /**