code: public static String getExtension(File file)
    {
      return file != null ? getExtension(file.getAbsolutePath()) : null;
    }
docs: Get file extension as lower case or empty string. Returned extension does not contain dot separator, that is,
  <code>htm</code> not <code>.htm</code>. Returns null if <code>file</code> parameter is null.
  
  @param file file to return extension of.
  @return file extension or empty string or null if <code>file</code> parameter is null.