Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 19726)
+++ wp-admin/includes/file.php	(working copy)
@@ -323,7 +323,7 @@
 
 	$filename = wp_unique_filename( $uploads['path'], $file['name'], $unique_filename_callback );
 
-	$tmp_file = wp_tempnam($filename);
+	$tmp_file = wp_tempnam( $file['name'], "{$uploads['path']}/" );
 
 	// Move the file to the uploads dir
 	if ( false === @ move_uploaded_file( $file['tmp_name'], $tmp_file ) )