There is only html mail but if you want to "hack" you can change global for all in the cls_universal_mailsender.php from the include folder.
Just go to the line 300
function UniversalMailSender($send_type = "")
{
if (empty($send_type) || $send_type == 1)
$this->mail_send_type = 1; // text
else
$this->mail_send_type = 2; // html
and change
$this->mail_send_type = 2;
to
$this->mail_send_type = 1; |