code: public static String getFileExtension(String _fileName) {
          if (_fileName == null) {
              return null;
          }
          int lastDot = _fileName.lastIndexOf('.');
          if (lastDot == -1) {
              return "";
          }
          return _fileName.substring(lastDot + 1);
      }
docs: Extracts the file extension (part behind last dot of a filename).
  Only returns the extension, without the leading dot.
  
  @param _fileName filename
  @return extension, empty string if not dot was found in filename or null if given String was null