Jquery.form 上传有关问题

JS function people_modif() {        var vURL = "admin_action.ashx?&rand=" + Math.random();        $("#peopledetail").ajaxSubmit({            url:encodeURI(vURL),             type: "post",            dataType: "text",            success: function (str) {                            },            error: function (error) {            }        });    }HTML:<div style=" float:left">更新图片：<input id="popmodifimage" type="file"  name="popmodifimage" /></div>

后台代码 action.ashxHttpPostedFile pf = Request.Files["popmodifimage"];if (pf != null){    string fileExtension = System.IO.Path.GetExtension(pf.FileName); //上传文件的扩展名    if (fileExtension != ".jpg" && fileExtension != ".gif" && fileExtension != ".bmp" && fileExtension != "png")    {        Response.Write("-1");        return;    }    fileName = DateTime.Now.ToString("yyyyMMddHHmmss") + fileExtension; //给文件重新命名    pf.SaveAs(path + fileName);    if (mpopimgurl != "") File.Delete(path + mpopimgurl);    strimg = " ,ImageUrl=@imageurl ";}