code: public function setContent($data, $mime, $charset=NULL) {
  		if (!$charset)
  			$charset=$this->charset;
  		$this->message[$mime] = [
  			'content'=>$data,
  			'type'=>$mime.'; '.$charset
  		];
  	}
docs: set message contents by mime type
  @param string $data message data
  @param string $mime the mime type
  @param null $charset