private function buildMultiPart()
{
$multipart = array();
foreach ($this->files as $field => $file) {
$multipart[] = ['name' => $field, 'contents' => fopen($file, 'r')];
}
foreach ($this->form_params as $param => $value) {
$multipart[] = ['name' => $param, 'contents' => $value];
}
return $multipart;
}