code: public function attachment($data = null, $mimeType = 'application/octet-stream')
      {
          if ($data !== null) {
              if (is_array($data)) {
                  $this->attachment['data'] = array_key_exists('data', $data) ? $data['data'] : null;
                  $this->attachment['mimeType'] = array_key_exists('mimeType', $data) ? $data['mimeType'] : 'application/octet-stream';
              } else {
                  $this->attachment['data'] = $data;
                  $this->attachment['mimeType'] = $mimeType;
              }
          }
  
          return $this->attachment;
      }
docs: Method to get or set the object's attachment. Overrides any existing attachment is setting.
  
  @param mixed $data If a string it is assumed to be the contents of the attachment. If an array copy its values across.
  @param string $mimeType The MIME type of the attachment that will be used in the request. Defaults to application/octet-stream.
  
  @return mixed Returns the contents of the current atachment or null if none has been specified.