jquery.from.js提交表单不选择文件报400

//提交数据
$(function(){
     var bar = $('.bar');
        var percent = $('.percent');
        var status = $('#status');
        $('form').ajaxForm({
            beforeSerialize:function(){
                //alert("表单数据序列化前执行的操作！");
                //$("#txt2").val("java");//如：改变元素的值
            },
            beforeSubmit:function(){
                //alert("表单提交前的操作");
                var file =$("input[type='file']")[0].files[0];
                if(typeof(file) == "undefined"){
                 return true;
                }
                var filesize = file.size/1024/1024;
                if(filesize > 10){
                  alert("文件大小超过限制，最多10");
                  return false;
                }
                //if($("#txt1").val()==""){return false;}//如：验证表单数据是否为空
            },
            beforeSend: function() {
                status.empty();
                var percentVal = '0%';
                bar.width(percentVal)
                percent.html(percentVal);
                
            },
            uploadProgress: function(event, position, total, percentComplete) {//上传的过程
                //position 已上传了多少
                //total 总大小
                //已上传的百分数
                var percentVal = percentComplete + '%';
                bar.width(percentVal)
                percent.html(percentVal);
                //console.log(percentVal, position, total);
            },
            success: function(data) {//成功
                var percentVal = '100%';
                bar.width(percentVal)
                percent.html(percentVal);
                if (data) {
swal({
title : "完成",
text : "编辑礼物成功",
type : "success"
},
function() {
var index = parent.layer
.getFrameIndex(window.name);
parent.layer.close(index);
});
} else {
swal({
title : "失败！",
text : "系统异常",
type : "success"
});
}
                
            },
            error:function(err){//失败
             console.log(err)
                alert("表单提交异常！"+err.msg);
            },
            complete: function(xhr) {//完成
                status.html(xhr.responseText);
            }
        });     });