docs: public static String extName(String fileName) {
  		if (fileName == null) {
  			return null;
  		}
  		int index = fileName.lastIndexOf(StrUtil.DOT);
  		if (index == -1) {
  			return StrUtil.EMPTY;
  		} else {
  			String ext = fileName.substring(index + 1);
  			// 扩展名中不能包含路径相关的符号
  			return StrUtil.containsAny(ext, UNIX_SEPARATOR, WINDOWS_SEPARATOR) ? StrUtil.EMPTY : ext;
  		}
  	}