code: protected static String extractFileExtension(String fileName) {
  		checkNotNull(fileName);
  		int lastPeriodIndex = fileName.lastIndexOf('.');
  		if (lastPeriodIndex < 0){
  			return null;
  		} else {
  			return fileName.substring(lastPeriodIndex + 1);
  		}
  	}
docs: Returns the extension of a file name (!= a path).
  @return the extension of the file name or {@code null} if there is no extension.