code: @Nullable
    private static String extractBase64String(String url) {
      if (url.startsWith(BASE64_URL_PREFIX) && url.contains(BASE64_START)) {
        int base64StartIndex = url.indexOf(BASE64_START);
        String mediaType = url.substring(BASE64_URL_PREFIX.length(), base64StartIndex);
        if (ACCEPTED_MEDIA_TYPES.contains(mediaType)) {
          byte[] data = BaseEncoding.base64().decode(
                  url.substring(base64StartIndex + BASE64_START.length()));
          return new String(data, UTF_8);
        }
      }
      return null;
    }
docs: Based on https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs
  @param url
  @return String or null.