blueimp jquery file upload - "done", "complete" callbacks not working for IE 9

I am using Blueimp Jquery File Upload plugin to upload files asynchronously. It works well in most other browsers (with a few minor issues) - on IE, I see this issue that the "done", "stop", "always", "complete" and some other event callbacks are not getting invoked.

While debugging, I added console.logs in the "done", "fail", "always", and added a "complete" method to the ajax request in the _onSend function (in jquery.fileupload.js) - but none of them seem to get invoked in IE.

_onSend: function (e, data) {
        var that = this,
            jqXHR,
            slot,
            pipe,
            options = that._getAJAXSettings(data),
            send = function (resolve, args) {
                that._sending += 1;
                jqXHR = jqXHR || (
                    (resolve !== false &&
                    that._trigger('send', e, options) !== false &&
                    (that._chunkedUpload(options) || $.ajax(options))) ||
                    that._getXHRPromise(false, options.context, args)
                ).complete(function (result, textStatus, jqXHR) {
                    console.log("complete"); 
                }).done(function (result, textStatus, jqXHR) {
                    console.log("done", result); 
                }).fail(function (jqXHR, textStatus, errorThrown) {
                    console.log("fail", result); 
                }).always(function (a1, a2, a3) {
                    console.log("done", result); 

                   }
                });
                return jqXHR;
            };

[plugin code trimmed for readability]

I understand that in IE 9, jquery.iframe-transport.js used for the file upload (as XHR file uploads are not supported in IE).

I'm not sure how I should go about fixing/ debugging this issue.

Thanks!

The done event gets fired if the content-type of the response is set to "text/html" or "text/plain" (instead of application/json) when json is being returned from the server. This applies only for browsers that do not support XHR file upload (such as IE9) and where the blueimp plugin is using IFrame transport instead.

Related info under "Content Negotiation" in the plugin documentation: https://github.com/blueimp/jQuery-File-Upload/wiki/Setup