code: private String extractExtension(String filename) {
      int startExtension = filename.indexOf('.');
      if (startExtension > 0) {
        return filename.substring(startExtension + 1);
      }
      return "";
    }
docs: A filename, for example, one3-attempt-01.txt.gz will return txt.gz
  
  @param filename The path of filename to extract the extension from
  @return The extension of the filename