If all the app need is to upload a photo of PCB, <input type="file"> is more than sufficient. It's been baseline years ago.

For download, it can download from a blob URI. This is not an uncommon practice.

If (not verified since I'm using Firefox) it claims that "Gerber files are composed of many individual files so that those two don't suffice" and the app does involve Gerber processing, it could have been solved by introducing a zip library.

"Gerbers" are indeed several individual files -- there's one for each layer of the PCB, such as front copper, front solder mask, front silkscreen, back copper, back solder mask, etc, etc.

A zip library is precisely how other webapps that load or output Gerbers handle it.