Index: wp-includes/ms-files.php
===================================================================
--- wp-includes/ms-files.php	(revision 18557)
+++ wp-includes/ms-files.php	(working copy)
@@ -23,7 +23,7 @@
 	die( '404 — File not found.' );
 }
 
-$file = BLOGUPLOADDIR . str_replace( '..', '', $_GET[ 'file' ] );
+$file = trailingslashit( BLOGUPLOADDIR ) . str_replace( '..', '', $_GET[ 'file' ] );
 if ( !is_file( $file ) ) {
 	status_header( 404 );
 	die( '404 — File not found.' );