request_var doesn't work correctly with umlauts

I wrote a small php download script for my website but there are problems with umlauts (ä, ü, ö). I use the request_var function to fetch the parameters. All works correctly without them but if there is an umlaut

$file = request_var('file', '');

returns ''; I tried to enable multibyte but nothing worked. If I use $_GET["file"] instead all is working correctly.