使用spring mvc上传，但MultipartFile为null

Question

 @ResponseBody
    @RequestMapping(value = "companyTypeRelation/toupload")
    public String uploadCompanyTypeRelation(
            @RequestParam(value = "uploadfile", required = false) MultipartFile[] uploadfile,
            HttpServletRequest request,HttpServletResponse response) {
          // do logic  
}

我使用ajax发送;但我发现MultipartFile为null，我可以得到该值;我不知道如何解决它？ 下面是jsp详细信息：

<div class="upload" id="uploadDIV" title="upload"
    style="text-align: center; display: none; padding: 5px;">
    <center>
    <a href="${pageContext.request.contextPath }/report/Excels/blalist.xls">uploadupload</a>
        <form
            action="${pageContext.request.contextPath }/backend/basic/companyTypeRelation/toupload"
            id="UploadForm" method="post" enctype="multipart/form-data">

            <br />
            <input  type="file" id="uploadfile" name="uploadfile"></input>
        </form>

    </center>
</div>

我使用ajax发送文件格式到服务器，也许有问题，但我不明白：

enter code here
$("#uploadDIV").dialog({
                            autoOpen : false,
                            height : 200,
                            width : 450,
                            modal : true,
                            buttons : {
                                "commit" : function() {
                                    var filepath = document
                                            .getElementById("uploadfile").value;
                                    var re_text = /\xls/i;
                                    var newFileName = filepath.split('.');
                                    newFileName = newFileName[newFileName.length - 1];
                                    if (newFileName.search(re_text) == -1) {

                                        return;
                                    }
                                    var resourceURL = '${pageContext.request.contextPath}/backend/basic/companyTypeRelation/toupload';
                                    //$("#UploadForm").submit();
                                    $.ajax({
                                        url:resourceURL,
                                        type: 'POST',
                                        data: {

                                            },
                                        error: function(XMLHttpRequest, textStatus, errorThrown){

                                            $(".validateTips").text("...");
                                        },
                                        success: function(data){
                                            alert(data);
                                        }
                                    });

                                },
                                "cancel" : function() {
                                    $(this).dialog("close");
                                }
                            },
                            close : function() {
                                allFields.val("").removeClass(
                                        "ui-state-error");
                            }
                        });