求助context.Request.Files["IptFiles"]为Null

页面代码如下<body>
    <form id="form1" runat="server" enctype="multipart/form-data" method="post" 
        action="~/jqueryApp/jqueryApp_UploadImg/UpLoadImg.ashx">
    <div>
        <p><img alt="图片" id="Img1" width="100" height="100" src="Image/defultImg.png"/></p>
        <p><input type="button" value="上传图片" id="IptUpload" style="width:100px" onclick="Fn_UpLoad()" /></p>
        <p><input type="file" name="IptFiles" style="display:none" id="IptFiles" /></p>
    </div>
    </form>
    <script type="text/javascript" src="jquery/jquery-1.4.2.min.js"></script>
</body>
</html>
<script type="text/javascript">
    function Fn_UpLoad() {
        $("#IptFiles").click();
        $("#form1").submit();
        var FileName = $("#IptFiles").val();
        $.getJSON("UpLoadImg.ashx", { Option: "SaveImg" }, function(json) {
            if (json[0].Result == "Yes") { 
            
            }
        });
    }
</script>ashx代码如下  string Option = context.Request.QueryString["Option"].ToString();
        if (Option == "SaveImg")
        {
            HttpPostedFile UploadFile = context.Request.Files["IptFiles"];
            string fileName = UploadFile.FileName;
            string FileType = UploadFile.FileName.Substring(UploadFile.FileName.LastIndexOf("."));
            if (fileName.Length > 0)
            {
                string prefix = DateTime.Now.ToString("yyyMMddHHmmss");
                fileName = context.Server.MapPath("~/jqueryApp/jqueryApp_UploadImg/Image/" + prefix+FileType);
                UploadFile.SaveAs(fileName);
                strMsg.Append("[{\"Result\":\"1\",\"ImgPath\":\"" + fileName + "}]");
            }
        }