<?php
$files = array(
'QDDWNiQDWiudn' => '../../downloads/calc.exe'
);
if($files[$_SERVER['QUERY_STRING']]) {
Header('Content-Type: application/octet-stream; name="'.basename($files[$_SERVER['QUERY_STRING']]).'"');
echo file_get_contents($files[$_SERVER['QUERY_STRING']]);
} else echo 'Нет такого файла :P';
?>