Aborting the xmlhttprequest

addEventListener will set the context (this) of uploadCanceled to xhr:

function uploadCanceled(evt) {
    console.log("Cancelled: " + this.status);
}

Example: http://jsfiddle.net/wJt8A/

---

If, instead, you need to trigger xhr.abort through a "Cancel" click, you can return a reference and add any listeners you need after that:

function uploadFile() {
    /* snip */
    xhr.send(fd);

    return xhr;
}

document.getElementById('submit').addEventListener('click', function () {
    var xhr = uploadFile(),
        submit = this,
        cancel = document.getElementById('cancel');

    function detach() {
        // remove listeners after they become irrelevant
        submit.removeEventListener('click', canceling, false);
        cancel.removeEventListener('click', canceling, false);
    }

    function canceling() {
        detach();
        xhr.abort();
    }

    // detach handlers if XHR finishes first
    xhr.addEventListener('load', detach, false);

    // cancel if "Submit" is clicked again before XHR finishes
    submit.addEventListener('click', canceling, false);

    // and, of course, cancel if "Cancel" is clicked
    cancel.addEventListener('click', canceling, false);
}, false);

Example: http://jsfiddle.net/rC63r/1/

You should be able to reference the "this" keyword in your canceledUpload event handler. That refers to the XMLHttpRequest. Put this in the handler:

this.abort();