this.uploadQueueLoadNext();
 };
 
+DDFileUploaderBase.prototype.progressHandlerCB = function(progress) {
+       // To be implemented by descendant.
+       // 0 <= progress <= 1
+};
+
 DDFileUploaderBase.prototype.progressHandler = function(evt) {
        if (evt.lengthComputable) {
                var progress = evt.loaded / evt.total;
-               this.updateProgressBar(progress);
-               var currentOpacity = this.previewImg.style.opacity;
-               this.previewImg.style.opacity = Math.max(currentOpacity, progress);
+               this.progressHandlerCB(progress);
        }
 };