A working PHP script that uses a local file as an attachment via the Web API.

webapiattup.php

<?php
$url = 'http://sendgrid.com/';
$user = '******';
$pass = '******';

$fileName = 'music.mp3';
$filePath = dirname(__FILE__);

$params = array(
    'api_user'  => $user,
    'api_key'   => $pass,
    'to'        => '[email protected]',
    'subject'   => 'test of file sends',
    'html'      => '<p> the HTML </p>',
    'text'      => 'the plain text',
    'from'      => '[email protected]',
    'files['.$fileName.']' => '@'.$filePath.'/'.$fileName
  );

print_r($params);

$request =  $url.'api/mail.send.json';

// Generate curl request
$session = curl_init($request);

// Tell curl to use HTTP POST
curl_setopt ($session, CURLOPT_POST, true);

// Tell curl that this is the body of the POST
curl_setopt ($session, CURLOPT_POSTFIELDS, $params);

// Tell curl not to return headers, but do return the response
curl_setopt($session, CURLOPT_HEADER, false);
curl_setopt($session, CURLOPT_RETURNTRANSFER, true);

// obtain response
$response = curl_exec($session);
curl_close($session);

// print everything out
print_r($response);
?>