I usually use something like this to force file uploads. Do not forget to change the MIME type if you change the type of the downloaded file.
$attachment_location = $_SERVER["DOCUMENT_ROOT"] . "/file.zip";
if (file_exists($attachment_location)) {
header($_SERVER["SERVER_PROTOCOL"] . " 200 OK");
header("Cache-Control: public"); // needed for i.e.
header("Content-Type: application/zip");
header("Content-Transfer-Encoding: Binary");
header("Content-Length:".filesize($attachment_location));
header("Content-Disposition: attachment; filename=file.zip");
readfile($attachment_location);
die();
} else {
die("Error: File not found.");
}