Getting Uploadify to work with asp.net-mvc

I am trying to get Uploadify to work with my site but I am getting a generic "HTTP Error" even before the file is sent to the server (I say this because Fiddler does not show any post request to my controller.

I can browse correctly for the file to upload. The queue is correctly populated with the file to upload but when I hit on the submit button the element in the queue get a red color and say HTTP Error.

Anyway this is my partial code:

<% using ( Html.BeginForm( "Upload", "Document", FormMethod.Post, new { enctype = "multipart/form-data" } ) ) { %>
<link type="text/css" rel="Stylesheet" media="screen" href="/_assets/css/uploadify/uploadify.css" />
<script type="text/javascript" src="/_assets/js/uploadify/swfobject.js"></script>
<script type="text/javascript" src="/_assets/js/uploadify/jquery.uploadify.v2.1.0.min.js"></script>
<script type="text/javascript">
    $(document).ready(function() {

        $("[ID$=uploadTabs]").tabs();

        var auth = "<% = Request.Cookies[FormsAuthentication.FormsCookieName]==null ? string.Empty : Request.Cookies[FormsAuthentication.FormsCookieName].Value %>";
        $('#fileInput').uploadify({
            uploader: '/_assets/swf/uploadify.swf',
            script: '/Document/Upload',
            folder: '/_uploads',
            cancelImg: '/_assets/images/cancel.png',
            auto: false,
            multi: false,
            scriptData: { token: auth },
            fileDesc: 'Any document type',
            fileExt: '*.doc;*.docx;*.xls;*.xlsx;*.pdf',
            sizeLimit: 5000000,
            scriptAccess: 'always', //testing locally. comment before deploy
            buttonText: 'Browse...'
        });

        $("#btnSave").button().click(function(event) {
            event.preventDefault();
            $('#fileInput').uploadifyUpload();
        });

    });
</script>
    <div id="uploadTabs">
        <ul>
            <li><a href="#u-tabs-1">Upload file</a></li>
        </ul>
        <div id="u-tabs-1">
            <div>
            <input id="fileInput" name="fileInput" type="file" />
            </div>
            <div style="text-align:right;padding:20px 0px 0px 0px;">
                <input type="submit" id="btnSave" value="Upload file" />
            </div>
        </div>
    </div>
<% } %>

Thanks very much for helping!

UPDATE:

I have added a "onError" handler to the uploadify script to explore which error was happening as in the following sample

onError: function(event, queueID, fileObj, errorObj) {
    alert("Error!!! Type: [" + errorObj.type + "] Info [" + errorObj.info + "]");
}

and discovered that the info property contains 302. I have also added the "method" parameter to uploadify with the value of 'post'.

I am including my controller action code for information. I have read many posts regarding uloadify and it seems that I can use an action with the following signature...

[HttpPost]
public ActionResult Upload(string token, HttpPostedFileBase fileData) {
    FormsAuthenticationTicket ticket = FormsAuthentication.Decrypt(token);
    if (ticket!=null) {
        var identity = new FormsIdentity(ticket);
        if(identity.IsAuthenticated) {
            try {
                //Save file and other code removed
                return Content( "File uploaded successfully!" );
            }
            catch ( Exception ex ) {
                return Content( "Error uploading file: " + ex.Message );
            }
        }
    }
    throw new InvalidOperationException("The user is not authenticated.");
}

Can anybody provide some help please?