var fileName = "wanjunshijiecom 2022-02-13 at 20.38.28.jpg";

方法1

var fileExtension = fileName.substring(fileName.lastIndexOf('.') + 1);


方法2

var fileExtension =fileName.substring(fileName.length-3,fileName.length).toLowerCase();

方法3

var fileExtension = fileName.split('.').pop().toLowerCase();