Multsite: Flush output buffer after readfile() in ms-files.php.

Props fuscata, MikeLittle for testing, iamfriendly for testing.
Fixes #14730.


git-svn-id: https://develop.svn.wordpress.org/trunk@38662 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-files.php

@@ -80,3 +80,4 @@ if ( ( $client_last_modified && $client_etag )
 
 // If we made it this far, just serve the file
 readfile( $file );
+flush();