code: public static String getExtension(String fileName) {
          if (fileName == null)
              return null;
          if (fileName.isEmpty())
              return "";
  
          int idx = fileName.lastIndexOf(".");
          if (idx != -1) {
              return fileName.substring(idx);
          } else {
              return "";
          }
      }
docs: 获取文件后缀名, 包含 .
  
  @param fileName
  @return