protected function setMessageType()
{
$type = [];
if ($this->alternativeExists()) {
$type[] = 'alt';
}
if ($this->inlineImageExists()) {
$type[] = 'inline';
}
if ($this->attachmentExists()) {
$type[] = 'attach';
}
$this->message_type = implode('_', $type);
if ('' == $this->message_type) {
$this->message_type = 'plain';
}
}