callbackProxy = function(loaded, total) {
            if (loaded !== lastLoadedSent || total !== lastTotalSent) {
                callback(loaded, total);
            }

            lastLoadedSent = loaded;
            lastTotalSent = total;
        }
Keeps a running tally of total upload progress for a batch of files.

@param callback Invoked when total progress changes, passing calculated total loaded & total size values.
@param getSize Function that returns the size of a file given its ID
@constructor