code: func (b *Bot) Download(file *File, localFilename string) error {
  	reader, err := b.GetFile(file)
  	if err != nil {
  		return wrapSystem(err)
  	}
  	defer reader.Close()
  
  	out, err := os.Create(localFilename)
  	if err != nil {
  		return wrapSystem(err)
  	}
  	defer out.Close()
  
  	_, err = io.Copy(out, reader)
  	if err != nil {
  		return wrapSystem(err)
  	}
  	file.FileLocal = localFilename
  
  	return nil
  }
docs: // Download saves the file from Telegram servers locally.
  //
  // Maximum file size to download is 20 MB.