code: @Override
    public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
      GcsFileOptions instance = GcsFileOptions.getDefaultInstance();
      GcsFilename fileName = getFileName(req);
      GcsOutputChannel outputChannel;
      outputChannel = gcsService.createOrReplace(fileName, instance);
      copy(req.getInputStream(), Channels.newOutputStream(outputChannel));
    }
docs: [START doPost]