Comment by jmconfuzeus

Progress bars and previews are simply a matter of attaching an events to the http request and file input respectively.

When someone submits a form and navigates back, don't you just re-display the empty form?

If it needs to be a form bounded with the uploaded file (maybe as part of a wizard), then why not ask the server for it instead of storing client side state?

Maybe using Javascript without a SPA looks hard because you're trying to avoid storing state on the server as well as bypass native browser features.