JQuery :: File Upload - AjaxSubmit Sends Response To Wrong Window In IE

I am using ajaxSubmit from the jquery.form plugin for a file upload. Obviously my form contains a file input element, and therefore it needs to be submitted to an iframe and have its response read from the iframe (since xhr won't work with a file input). This is built into the jquery.form plugin and is working fine in firefox. However, when I submit my upload with IE, it is still acting like the form was submitted to the main page -- not the iframe. The response is just some JSON string indicating the results of the file upload.

In firefox, the upload runs, the json response comes back to the iframe, and then the callback reads the response from the iframe and updates the page. In IE, the upload runs and then the json response comes back to the main page as a text file...so IE pops up a window asking me if I want to download or open the file. Does anyone know why my JSON response is coming back to the main page and not the iframe?

Here is some of the code. The only semi-funky thing that goes on here is that the form is not set with a multipart encoding originally. This is because the main page's form submit should not be multipart. I set the form as multipart right before the fileUpload, and then before the main form is saved I reset the encoding. As far as I can tell this part is working, though...since the file is uploading no, problem; it's just the response that isn't being read correctly (or directed correctly?)

Here is a code snippet from the code that runs when the upload button is clicked.

// set the encoding to multipart. I've heard IE can be wonky, so it's set in two ways.
form.setAttribute('encoding', 'multipart/form-data');
form.setAttribute('enctype', 'multipart/form-data');

[Code]....