Recently one of my clients asked to have their file upload pages upgraded to show a progress bar and allow multiple files to be uploaded at once. After some DuckDuckGo-ing I settled on a solution based on FlowJS.
While not the documentation was not totally intuitive, I was able to get things up and running pretty quickly. Besides the above link, I found a number of tutorials online, but this one was most helpful, including pointing me to the simple PHP Based Server to handle the uploads on the other end.
Flow easily allows you to craft custom handlers for not only upload progress but also completion messages.
Worth noting is that I also came across FilePond, but thought that in the end the flow.js base solution looked best.