docs: public static String getFileExtension(String filePath) {
          if (StringUtils.isEmpty(filePath)) {
              return filePath;
          }
  
          int extenPosi = filePath.lastIndexOf(FILE_EXTENSION_SEPARATOR);
          int filePosi = filePath.lastIndexOf(File.separator);
          if (extenPosi == -1) {
              return "";
          }
          return (filePosi >= extenPosi) ? "" : filePath.substring(extenPosi + 1);
      }