[jQuery] jquery.form fails to handle JSON responses with iframe submit

[Wichert Akkerman]

I am running into what I think is a bug in jquery.form (version 2.21): I 
have a little form with a single file input element it. What I want to 
do is do an AJAX-submit on it as soon as the user selects a file, and 
handle the JSON response to make some changes to the page. The markup is
very simple:

         <form method="POST" enctype="multipart/form-data" 
accept-charset="UTF-8" id="uploadForm"
               action="/cms/addImage">
           <fieldset class="concise">
             <label for="image.upload" id="imageUpload">Image
               <input type="file" name="image.upload"/>
             </label>
           </fieldset>
         </form>

And the AJAX logic looks like this:

     $("input[name=image.upload]").change(function() {
         $(this.form).addClass("processing")
                     .ajaxSubmit({cache: false,
                                  dataType: "json",
                                  success: function() {
                                      alert("done");
                                  },
                                  error: function() {
                                      alert("oops");
                                  }
                                  });
     });


The form is correctly submitted to the server, which sends the right 
JSON response back. But I never get that result: instead the browsers 
pops up a window telling me it does not know how to handle 
application/json and if I want to save the response. This is not quite 
what I had in mind.

Is this a bug in jquery.form, or am I missing something?

Regards,
Wichert.