curl and PHP send data and attachment with Gravity Form

Question

first I've got a Wordpress Page with Gravity Form. on my functions.php is a function to send data to an extern page with curl and php. There is an array with my post field data and everything works great. BUT I want to send my attachments with it. and there is any problem.

in my function looks the curl part like this:

$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, 'https://playground.europersonal.com/api/public/v1/Bewerbung/Create');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
$post = array(
    'bewerbung' => json_encode($data),
    'attachments' => @https://domain.de/file.pdf'
);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);

$headers = array();
$headers[] = 'Accept: text/plain';
$headers[] = 'X-Apikey: xxxxx';
$headers[] = 'Content-Type: multipart/form-data';
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

$result = curl_exec($ch);
if (curl_errno($ch)) {
    echo 'Error:' . curl_error($ch);
}
curl_close($ch);

the attachment are not sending. I don't know how to get one or more files from my gravityform upload to the "attachments" array for my curl upload.

Anyone can help?