Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 11614)
+++ wp-admin/includes/file.php	(working copy)
@@ -167,9 +167,7 @@
 function wp_tempnam($filename = '', $dir = ''){
 	if ( empty($dir) )
 		$dir = get_temp_dir();
-	$filename = basename($filename);
-	if ( empty($filename) )
-		$filename = time();
+	$filename = md5($filename . time()) . '.tmp';
 
 	$filename = $dir . wp_unique_filename($dir, $filename);
 	touch($filename);