code: func (b *Bsteam) handleFileInfo(msg *config.Message, f interface{}) error {
  	if _, ok := f.(config.FileInfo); !ok {
  		return fmt.Errorf("handleFileInfo cast failed %#v", f)
  	}
  	fi := f.(config.FileInfo)
  	if fi.Comment != "" {
  		msg.Text += fi.Comment + ": "
  	}
  	if fi.URL != "" {
  		msg.Text = fi.URL
  		if fi.Comment != "" {
  			msg.Text = fi.Comment + ": " + fi.URL
  		}
  	}
  	return nil
  }
docs: // handleFileInfo handles config.FileInfo and adds correct file comment or URL to msg.Text.
  // Returns error if cast fails.