docs: private GcsFilename getFileName(HttpServletRequest req) {
      String[] splits = req.getRequestURI().split("/", 4);
      if (!splits[0].equals("") || !splits[1].equals("gcs")) {
        throw new IllegalArgumentException("The URL is not formed as expected. " +
            "Expecting /gcs/<bucket>/<object>");
      }
      return new GcsFilename(splits[2], splits[3]);
    }