download(filepath, filename = path.basename(filepath)) {
    assert(filepath, 'filepath can not be empty');
    const contentType = this.response.get('Content-Type');
    if (!contentType) {
      this.type = path.extname(filename);
    }
    const contentDisposition = this.response.get('Content-Disposition');
    if (!contentDisposition) {
      this.attachment(filename);
    }
    const stream = fs.createReadStream(filepath);
    this.body = stream;
    onFinished(this.res, () => {
      destroy(stream);
    });
    return false;
  }
download
@param {String} filepath
@param {String} filename