private function buildAttachmentPart() {
    if (count($this->attachments) > 0) {
      $attachment_part = '';
      foreach ($this->attachments as $attachment) {
        $file_str = chunk_split(base64_encode(file_get_contents($attachment)));
        $attachment_part .= "--MIME_BOUNDRY\nContent-Type: ".$this->getMimeType($attachment)."; name=".basename($attachment)."\nContent-disposition: attachment\nContent-Transfer-Encoding: base64\n\n{$file_str}\n\n";
      }
      return $attachment_part;
    }
  }