code: func (c *Client) DownloadFile(ctx context.Context, file string, u *url.URL, param *Download) error {
  	var err error
  	if param == nil {
  		param = &DefaultDownload
  	}
  
  	rc, contentLength, err := c.Download(ctx, u, param)
  	if err != nil {
  		return err
  	}
  
  	return c.WriteFile(ctx, file, rc, contentLength, param.Progress, param.Writer)
  }
docs: // DownloadFile GETs the given URL to a local file