code: func (d *DirectoryUploader) Upload(upload UploadFn) error {
  	return filepath.Walk(d.Dir, func(path string, f os.FileInfo, err error) error {
  		if err != nil {
  			return err
  		}
  		if f.IsDir() {
  			return nil
  		}
  		file, err := Open(path)
  		if err != nil {
  			return err
  		}
  		relPath, err := filepath.Rel(d.Dir, path)
  		if err != nil {
  			return err
  		}
  		file.Path = filepath.ToSlash(relPath)
  		return upload(file)
  	})
  }
docs: // Upload performs the upload of the directory and default path