I had a support communication with the team and I want to give to all of you easy way how to do it.
In the helper_functions.php just before this line
$mail->Body = $email_content;
add this line:
$mail->AddAttachment('./data/form_'.$form_id.'/files/'.'filename.ext');
where $form_id is the id of the form. You can use this parameter so all the forms can have some file to attach. If you want to separate the names of the files for each form you can use switch command.
switch($form_id){
case 1 ...
...
}
You can also avoid sending attachment to the emails to notification if it is on.
if (!$target_is_admin) ...