code: func firstFile(tr *tar.Reader) ([]byte, error) {
  	if _, err := tr.Next(); err != nil {
  		return nil, err
  	}
  
  	var buf bytes.Buffer
  	if _, err := io.Copy(&buf, tr); err != nil {
  		return nil, err
  	}
  
  	return buf.Bytes(), nil
  }
docs: // firstFile extracts the first file from a tar archive.