code: func (f *FileSystem) Open(p string) (http.File, error) {
  	p = path.Clean(p)
  
  	if len(f.LocalPath) != 0 {
  		return http.Dir(f.LocalPath).Open(p)
  	}
  
  	if fi, ok := f.Files[p]; ok {
  		if !fi.IsDir() {
  			// Make a copy for reading
  			ret := fi
  			ret.buf = bytes.NewReader(ret.Data)
  
  			return ret, nil
  		}
  
  		return fi, nil
  	}
  
  	return nil, os.ErrNotExist
  }
docs: // Implementation of http.FileSystem