字符串：var s = "1,2,3,4,5,"

目标：删除最后一个 ","

方法：
s=s.substring(0,s.Length-1)；

字符串：var s2 = "img/upload/123.jpg"

目标：获取文件名(不包含后缀名)

方法：
s2=s2.substring(s2.lastIndexOf('/')+1, s2.lastIndexOf('.'));