How to Post File

Hi,
I'm using webform_php module for additional processing. I send form vaules to second server. For this, i use code:

<?php
$url = 'http://example.com/example.php';
$headers = array('Content-Type' => 'application/x-www-form-urlencoded; charset=UTF-8');
$data = http_build_query($form_values['submitted_tree'], '', '&');
$req = drupal_http_request($url, $headers, 'POST', $data);
?>

But don't send file field content. How to send uploaded files to other server?

Thanks.