PHPMailer::setMessageType PHP Method

    protected function setMessageType()
    {
        $type = array();
        if ($this->alternativeExists()) {
            $type[] = 'alt';
        }
        if ($this->inlineImageExists()) {
            $type[] = 'inline';
        }
        if ($this->attachmentExists()) {
            $type[] = 'attach';
        }
        $this->message_type = implode('_', $type);
        if ($this->message_type == '') {
            //The 'plain' message_type refers to the message having a single body element, not that it is plain-text
            $this->message_type = 'plain';
        }
    }