From http://stackoverflow.com/questions/18318052/php-curl-post-with-file-attachement-custom-content-type-header

SendFileCurl.php

<?php
$url     = 'https://hostserver.com/gateway/remote_send';
$payload = array(
    'profile_name'  => 'username',
    'profile_pw'    => 'password1234',
    'attached_type' => 'action_1'
);
$file = realpath('/home/username/tests/test1234qwerty.csv');

// build multipart
$payload = http_build_query($payload);
$params  = "--ABC1234\r\n"
    . "Content-Type: application/x-www-form-urlencoded\r\n"
    . "\r\n"
    . $payload . "\r\n"
    . "--ABC1234\r\n"
    . "Content-Type: text/csv\r\n"
    . "Content-Disposition: attachment; filename=\"attachment.csv\"\r\n"
    . "\r\n"
    . file_get_contents($file) . "\r\n"
    . "--ABC1234--";

$first_newline      = strpos($params, "\r\n");
$multipart_boundary = substr($params, 2, $first_newline - 2);
$request_headers    = array();
$request_headers[]  = 'Content-Length: ' . strlen($params);
$request_headers[]  = 'Content-Type: multipart/x-api-remote-integration; boundary='
    . $multipart_boundary;

// send the request now

$ch = curl_init($url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $params);
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_0);
curl_setopt($ch, CURLOPT_HTTPHEADER, $request_headers);

$reply = curl_exec($ch);