IE prompts to open or save json result from server

Even though it's not supposedly the correct way, setting the content type to text/html made IE deal with this correctly for me:

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

Works in all the version that F12 tools gives you in IE9.

If using MVC, one way of handling this is to implement a base controller in which you override (hide) the Json(object) method as follows:

public class ExtendedController : Controller
{
    protected new JsonResult Json(object data)
    {
        if (!Request.AcceptTypes.Contains("application/json"))
            return base.Json(data, "text/plain");
        else
            return base.Json(data);
    }
}

Now, your controllers can all inherit ExtendedController and simply call return Json(model); ...

• without modifying the response content type for those browsers which play nicely (not <=IE9 !)
• without having to remember to use Json(data, "text/plain") in your various Ajax action methods

This works with json requests which would otherwise display the "Open or Save" message in IE8 & IE9 such as those made by jQuery File Upload

Sounds like this SO question may be relevant to you:

How can I convince IE to simply display Application json rather than offer to download

If not:

Have you tried setting the dataType expected in the ajax options? i.e. dataType: 'json'

Have you tried other content types such as 'application/json' or 'text/javascript'