$(".submitbtn").on("click", function(e) {

    var form = $("#Form");

    // you can't pass Jquery form it has to be javascript form object
    var formData = new FormData(form[0]);

    //if you only need to upload files then 
    //Grab the File upload control and append each file manually to FormData
    //var files = form.find("#fileupload")[0].files;

    //$.each(files, function() {
    //  var file = $(this);
    //  formData.append(file[0].name, file[0]);
    //});

    if ($(form).valid()) {
        $.ajax({
            type: "POST",
            url: $(form).prop("action"),
            //dataType: 'json', //not sure but works for me without this
            data: formData,
            contentType: false, //this is requireded please see answers above
            processData: false, //this is requireded please see answers above
            //cache: false, //not sure but works for me without this
            error   : ErrorHandler,
            success : successHandler
        });
    }
});