XenForo_FileOutput doesn't go via the dynamic class resolution. This means adding support for; nginx's X-Accel-Redirect (ref http://wiki.nginx.org/X-accel) the Apache module mod_xsendfile lighttpd's x-sendfile version. Requires a code edit, or a custom autoloader. These are advanced features, which allow php to-do authentication and then delegate to the webserver to actually serve the file. Would I don't expect XF to implement all those features in core, it would be nice to be able to implement them via an addon simply by hooking the XenForo_FileOutput class by event listeners in the usual way.