Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 11596)
+++ wp-admin/includes/file.php	(working copy)
@@ -170,7 +170,7 @@
 	$filename = basename($filename);
 	if ( empty($filename) )
 		$filename = time();
-
+	$filename = preg_replace('|\..*$|', '.tmp', $filename);
 	$filename = $dir . wp_unique_filename($dir, $filename);
 	touch($filename);
 	return $filename;