asp.net mvc dialog box mechanics at the end of the request

Question

<form action="/Home/JSONRequest" method="post" enctype="multipart/form-data" onsubmit="return submitForm(this)">

 <input type="file" name="file" id="file" /> 
   <input type="submit" name="submit" value="Submit" />


</form>

 <script type="text/javascript">
  function submitForm(frm) {
      var tdata = $(frm).serialize();


      $.ajax({
      url: "/Home/JSONRequest",
      data: tdata,

          success: function(result) {




          },
          type: "POST",
          datatype: "json"
      });
  }
  </script> 


    public ActionResult JSONRequest(FormCollection form)
    {

        string a = form["name"];
        HttpPostedFileBase upload = Request.Files["file"] as HttpPostedFileBase;
     var data = new { name = "aaaa", Success = "Record is Succesfully Saved"}

              return Json( data );  


    }

Can anybody tells me why there is a dialog box at the end of the flow of  the code asking to save the file. what i could check ,what is going 
wrong . This is really very frustating issue