people usually resort to running ajax requests in parallel to ask the server on a second connection how much data was received so far on the first connection

An alternative approach that I prefer is to use an AJAX request to perform the upload. Using XMLHttpRequest an event handler can be added to the request object to then show the progress something like this:

xhr.upload.addEventListener('progress', function(ev) {
    document.getElementById('progbar').value = ev.loaded * 100 / ev.to
+tal;
});
[download]

Where progbar is an HTML progress element.