code: public static String extension(String file) {
        if (StringUtils.isNullOrBlank(file)) {
           return StringUtils.EMPTY;
        }
        file = StringUtils.trim(file);
        int index = indexOfFileExtension(file);
        if (index == -1) {
           return StringUtils.EMPTY;
        }
        return file.substring(index + 1);
     }
docs: <p>Attempts to get the file extension for a given file.</p>
  
  @param file The file
  @return The file extension of the file spec or null if it is null