ADD_ATTACHMENT api request not working

Hi,

I'm trying to add an attachment to a support ticket already created. Please see my code below.

This is written in PHP and therefore I'm finding it difficult to understand the sample code given in documentation.

<?php
$fields = array('filename' => 'test_file.pdf;type=application/pdf');
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, ' http://servicedesk/sdpapi/request/ticketid/attachment?OPERATION_NAME=ADD_ATTACHMENT&TECHNICIAN_KEY=MY TECHNICIAN KEY');
curl_setopt($ch, CURLOPT_POST, TRUE);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($fields));
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: multipart/form-data'));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);

print_r($response);
?>

The error message I'm getting is "Error when adding attachment, the request was rejected because no multipart boundary was found"

Please let me know how this can be corrected.

Thanks!