js 获取input file上传文件大小

$("#shop_img").change(function (e) {
      var file, img;
      if ((file = this.files[0])) {
        img = new Image();
        img.onload = function () {
            if(this.width != 640 || this.height!=480){
                layer.open({
                    content:'请上传店铺照片大小为640*480',
                    skin:'msg',
                    time:3
                });
            }else{
                shop_img='11';
            }
        };
        img.src = _URL.createObjectURL(file);
      }
    });