docs: @Pure
  	public static String extension(File filename) {
  		if (filename == null) {
  			return null;
  		}
  		final String largeBasename = largeBasename(filename);
  		final int idx = largeBasename.lastIndexOf(getFileExtensionCharacter());
  		if (idx <= 0) {
  			return ""; //$NON-NLS-1$
  		}
  		return largeBasename.substring(idx);
  	}