plupload with ASP.NET forms authenticated handler-Collection of common programming errors

I am trying to use plupload to upload files in ASP.NET. My upload handler ideally needs to be included in the forms authentication on my site, but it doesn’t work in this case. I can’t see the request that plupload makes in firebug, and don’t really understand what its doing. Is there any way to get around this problem?

Here is my plupload initialisation code:

$(function () {
    $("#uploader").pluploadQueue({
        runtimes: 'gears,flash,silverlight,browserplus,html5,html4',
        url: '/Handlers/UploadImage.ashx',
        filters: [
            { title: "Image files", extensions: "jpg,gif,png" },
            { title: "Zip files", extensions: "zip" }
        ],
        flash_swf_url: '/Scripts/plupload/plupload.flash.swf',
        silverlight_xap_url: '/Scripts/plupload/plupload.silverlight.xap'
    });