Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 13947)
+++ wp-includes/functions.php	(working copy)
@@ -2209,7 +2209,7 @@
 		$name = '';
 
 	// Increment the file number until we have a unique file to save in $dir. Use $override['unique_filename_callback'] if supplied.
-	if ( $unique_filename_callback && function_exists( $unique_filename_callback ) ) {
+	if ( $unique_filename_callback && is_callable( $unique_filename_callback ) ) {
 		$filename = $unique_filename_callback( $dir, $name );
 	} else {
 		$number = '';