Objects of Mail class have their way of adding files as attachments.
static public function Send($id_lang, $template, $subject, $templateVars, $to, $toName = NULL, $from = NULL, $fromName = NULL, $fileAttachment = NULL, $modeSMTP = NULL, $templatePath = _PS_MAIL_DIR_)
This is the signature of the Send function for the Mail class.
You can check the code in /classes/Mail.php
Or here in github