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

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

Built from https://develop.svn.wordpress.org/trunk@38662


git-svn-id: http://core.svn.wordpress.org/trunk@38605 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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();

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-alpha-38661';
+$wp_version = '4.7-alpha-38662';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.