code: function initFile (file) {
    if (!file.oss) {
      file.oss = {};
      file.oss.headers = {};
      file.oss.path = file.relative.replace(/\\/g, '/');
    }
    return file;
  }
docs: init file oss hash
  @param  {Vinyl} file file object
  
  @return {Vinyl} file
  @api private