diff --git a/wp-includes/ms-files.php b/wp-includes/ms-files.php
index f5af8e0..5ec6d1d 100644
--- a/wp-includes/ms-files.php
+++ b/wp-includes/ms-files.php
@@ -29,14 +29,14 @@ if ( !is_file( $file ) ) {
 	die( '404 — File not found.' );
 }
 
-$mime = wp_check_filetype( $_SERVER[ 'REQUEST_URI' ] );
+$mime = wp_check_filetype( $file );
 if( false === $mime[ 'type' ] && function_exists( 'mime_content_type' ) )
 	$mime[ 'type' ] = mime_content_type( $file );
 
 if( $mime[ 'type' ] )
 	$mimetype = $mime[ 'type' ];
 else
-	$mimetype = 'image/' . substr( $_SERVER[ 'REQUEST_URI' ], strrpos( $_SERVER[ 'REQUEST_URI' ], '.' ) + 1 );
+	$mimetype = 'image/' . substr( $file, strrpos( $file, '.' ) + 1 );
 
 header( 'Content-type: ' . $mimetype ); // always send this
 if ( false === strpos( $_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS' ) )