[Plugin: W3 Total Cache] Download script fails because of W3-Total-Cache

I’m have a simple download script that fails if W3-Total-Cache is enabled. No matter what I do, my content-length setting is ignored and the download fails after a few bytes. It seems like W3-Total-Cache sends its own content-length header which seems to be why mine is ignored (usually set to 0).

I plan on extending this script for user permissions, etc. and therefore I need some wordpress functions. When I remove the line to load wordpress everything works as it should.

The usage of this code is:

yoursite.com/download.php?file=yourfile

<?php
date_default_timezone_set('Europe/Paris');

// Load WordPress.
require_once('wp-load.php');

$file = $_GET['file'];

$info = pathinfo($file);
$ext = $info['extension'];

switch ($ext) {
	case "gif":	$ctype="image/gif";	break;
	case "jpeg":
	case "jpg":	$ctype = "image/jpg"; break;
	case "png":	$ctype = "image/png"; break;
	case "pdf":	$ctype = "application/pdf"; break;
	case "zip":	$ctype = "application/zip";	break;
	case "txt":	$ctype = "text/plain"; break;
	default: $ctype = "x-type/subtype";
}

if (file_exists($file)) {
	$size = filesize($file);
	header("Content-Length: " . $size);
	header("Content-Type: " . $ctype);
	header("Content-Disposition: attachment; filename=" . $filename);

	ob_end_clean();
	readfile($file);
	exit();
} else {
	die("ERROR: File doesn't exist!");
}