code: @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;
    }
docs: Returns the cache file for the given url if it exists. Checks for both json and zip.
  Returns null if neither exist.