How to return response of type Object from ajax call?

Question

I'm using ajax to call a webmethod from my SOAP webservice. When I call my webservice in this way:

MyWebservice.MyWebMethod("test",LunchMyFunction);

function LunchMyFunction(response) {
   alert(response); // [object Object] 
   var gridView = $find('<%= MyGridView.ClientID %>');
   gridView.set_dataSource(response);
   gridView.dataBind();
}

The returned response will be [object Object]

But when I call it using Ajax in this way:

$.ajax({
         type: "POST",
         url: 'MyWebservice.asmx/MyWebMethod',
         data:{ testVal : "test" },
         success: function (response) {
            alert(response); // [object XMLDocument]
            var gridView = $find('<%= grdvUploadFiles.ClientID %>');
            gridView.set_dataSource(response);
            gridView.dataBind();
         },
         error: function (err) { alert(err.statusText); }
});

The returned response here is [object XMLDocument]. Now the problem is my gridView is filled properly in the first case, while in the second it is not filled, not even displayed. I really need to call my webservice using ajax call because I have to pass files to my webmethod to be uploaded on server, and those files can't be sent by the first method. So what is the best way to return object of type Object not XMLDocument?

Edit:

I have tried this solution which is returning Object but it is not uploading file, the counter in my webmethod is returning 0 which means that file could not be sent in this way:

var fileUpload = $("#FileUpload1").get(0);
var files = fileUpload.files;
var fileData = new FormData();
fileData.append(files[0].name, files[0]);
fileData.append('username', 'Test');

$.ajax({
       type: "POST",
       url: 'MyWebservice.asmx/MyWebMethod',
       data: JSON.stringify(fileData),
       contentType: "application/json; charset=utf-8",
       processData: false,
       dataType: "json",
       success: function (response) {
       alert(response); // [object Object]
       ....
});

The old one was :

$.ajax({
        url: 'MyWebservice.asmx/MyWebMethod',
        type: "POST",
        contentType: false,
        processData: false,
        data: fileData,
        success: function (response) {
           alert (response);  // response is [object XMLDocument]
           var gridView = $find('<%= grdvUploadFiles.ClientID %>');
           gridView.set_dataSource(response);
           // gridView could not be filled and the process stops here
           gridView.dataBind();
        }
 });