Index: wp-includes/ms-files.php
===================================================================
--- wp-includes/ms-files.php	(revision 22329)
+++ wp-includes/ms-files.php	(working copy)
@@ -52,7 +52,7 @@
 }
 
 $last_modified = gmdate( 'D, d M Y H:i:s', filemtime( $file ) );
-$etag = '"' . md5( $last_modified ) . '"';
+$etag = '"' . md5( $last_modified . filesize( $file ) ) . '"';
 header( "Last-Modified: $last_modified GMT" );
 header( 'ETag: ' . $etag );
 header( 'Expires: ' . gmdate( 'D, d M Y H:i:s', time() + 100000000 ) . ' GMT' );