jquery - WebService Return Xml result instead of json

Question

that is my code , and i am wonder that why my web service in .net 4.0 always return data as a xml format instead of json .

Webservice :

    [WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public FileModel HelloWorld()
{      
  FileModel model= new FileModel();
  model.Name = "peyman";

  return model;
}

jQuery :

I get files list in drop event with: var files = event.originalEvent.dataTransfer.files; and then call upload(files);

function upload(files) {

    var data = new FormData();

    jQuery.each(files, function(i, file) {
        data.append('file-' + i, file);
    });

    $.ajax({
        type : "POST",
        url : parameters.Url,
        contentType : false,
        processData : false,
        data : data,
        dataType : "json",
        success : function(msg) {
            alert(msg);
        },
        error : function() {
            alert("err");
        }
    });
}

Also i tried some thing like :

• [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
• Context.Response.ContentType = "application/json";
• Adding "application/json" or "text/json" to response header

but response content-type is always "text/xml; charset=utf-8"