gistfile1.php

<?php
// authentication
if (!authentication()) {
  exit;
}

$filepath = "/path/to/file";

$sizes = getimagesize($filepath);
if (!empty($sizes['mime'])) {
  $mime = $sizes['mime'];
} else {
  $mime = 'application/octet-stream';
}   
header('Content-Type: '.$mime);
header('Content-Size: '.filesize($filepath));

readfile($filepath); // or fpassthru(fopen($filepath, 'rb'));
exit;