无法在Ajax中调用“uploadProgress”

Question

 $.ajax({

            url: "${pageContext.request.contextPath}/something/somthing",         
            data: oMyForm,              
            type: "POST",               
            processData: false,            
            contentType: false,

         beforeSend: function () {       
                alert("beforeSend");                
                status.empty();
                var percentVal = '0%';
                bar.width(percentVal);
                percent.html(percentVal);

            },
            uploadProgress: function (event, position, total, percentComplete) {            
                alert("uploadProgress");                
                var percentVal = percentComplete + '%';
                bar.width(percentVal);
                percent.html(percentVal);

            },
            success: function () {              
                alert("success");               
                var percentVal = '100%';
                bar.width(percentVal);
                percent.html(percentVal);

            },
            complete: function (xhr) {              
                alert("xhr:" +xhr);             
                alert("complete");              
                status.html(xhr.responseText);
            }
        });

在这里，我可以从控制器获取一个文件，并在uploadprogress中设置警报，但它不会到来。你能解决这个问题吗？

提前谢谢