To add files, use POST requests with each file transferred as a separate parameter named attach (n), where n is any seed value (see code example below).
Copy $curl = curl_init();
curl_setopt($curl, CURLOPT_URL,'https://youraccount.worksection.com/api/admin/
v2/?action=post_task&id_project=PROJECT_ID&title=TASK_NAME&hash=HASH');
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, [
'attach[0]' = new cURLFile('path_to_file/local_file1.pdf',
'application/pdf','nice_name1.pdf'),
'attach[1]' = new cURLFile('path_to_file/local_file2.pdf',
'application/pdf','nice_name2.pdf'),
]);
$response = json_decode(curl_exec($curl), true);
curl_close($curl);