FileOutputStream out = new FileOutputStream(file);
        try {
          out.write(initialBytes);
        } finally {
          out.close();
        }
        return Files.asByteSink(file, FileWriteMode.APPEND);
      }
      return Files.asByteSink(file);
    }

    @Override
    public byte[] getExpected(byte[] bytes) {
      if (initialBytes == null) {
        return checkNotNull(bytes);