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';
}
}