IIRC there was a feud between Chrome developers who wanted raw file access and Firefox ones who didn't want it. W3C sided with FF, so we can't build proper desktop apps using PWA, even if WASM makes them quite fast these days. Now Chrome devs resuscitated the attempt with NFS API, so I'll grab my popcorn and watch their interaction one more time...
I’ve worked on the Native File System API on Chrome.
It’s available through Origin Trial at the moment.
That said, Web apps can currently emulate reading files and writing to disk.
Take a look at this library, which also works as a poly fill:
https://github.com/tomayac/browser-nativefs