I'm trying to use jquery.Ajax to post data to an ASP.NET MVC2 action method that returns a JsonResult. Everything works great except when the response gets back to the browser it is treated as a file download instead of being passed into the success handler. Here's my code:

Javascript:

 <script type="text/javascript">
        $(document).ready(function () {
            $("form[action$='CreateEnvelope']").submit(function () {
                $.ajax({
                    url: $(this).attr("action"),
                    type: "POST",
                    data: $(this).serialize(),
                    dataType: "json",
                    success: function (envelopeData) {
                        alert("test");
                    }
                });
            });
            return false;
        });
    </script>

Action method on controller:

public JsonResult CreateEnvelope(string envelopeTitle, string envelopeDescription)
    {
        //create an envelope object and return
        return Json(envelope);
    }

If I open the downloaded file the json is exactly what I'm looking for and the mime type is shown as application/json. What am I missing to make the jquery.ajax call receive the json returned?