jQuery ajaxForm returning .json file

Question

I've got a model creation form in rails which I also have returning JSON through ajax. My code so far look like:

$('#new_stem').ajaxForm({ //#new_stem is my form
  dataType: 'json',
  success: formSuccess
});

function formSuccess(stemObj) {
  //does stuff with stemObj
}

And I have a multipart form with a file uploader (but I'm not sure if that is relevant).

When I submit the form it works fine (my models are properly being created and renders as json), but instead of the json getting handled by the formSuccess function, it prompts a download for "stems.json" (the path to my stem creation action) in Firefox.

What would cause this to happen, and what could solve it? Not sure if this is part of the problem, but I don't have a submit button in my form, I have a link with a click handler that calls $('#new_stem).submit()

Thanks guys!

EDIT: Firebug tells me the header contains the following:

Etag        "b53e5247e7719cf6b1840e2c6e68781c"
Connection      Keep-Alive
Content-Type    application/json; charset=utf-8
Date        Mon, 03 May 2010 02:19:31 GMT
Server      WEBrick/1.3.1 (Ruby/1.8.7/2010-01-10)
X-Runtime       241570
Content-Length  265
Cache-Control   private, max-age=0, must-revalidate

plus a cookie header

Answer 1

To prevent browser to trigger download of .json file set Content-type header to "text/html".

PHP:

header("Content-type: text/html");

ASP.NET MVC:

return Json(obj, "text/html");

In javascript you need to parse text result, like this:

$(".addform").ajaxSubmit({
            url: "file.php",
            type: "POST",
            dataType: "text",
            iframe: true,
            success: function (text) {
                var data = $.parseJSON(text);
            },
            error: function (xmlRequest, textStatus, errorThrown) {
                alert(errorThrown);
            }
        });

Works perfectly.