docs: @Nullable
    private File getCachedFile(String url) throws FileNotFoundException {
      File jsonFile = new File(appContext.getCacheDir(), filenameForUrl(url, FileExtension.JSON, false));
      if (jsonFile.exists()) {
        return jsonFile;
      }
      File zipFile = new File(appContext.getCacheDir(), filenameForUrl(url, FileExtension.ZIP, false));
      if (zipFile.exists()) {
        return zipFile;
      }
      return null;
    }