Access for directly manipulating the file isn't necessary (perhaps if the files were enormous, but images and Gerber files aren't). One can upload/download files from a local web app just fine on firefox, and the WASM app can act on the file in memory with whatever APIs it wants.

> in memory

This, but it can also have the browser store it to disk by requesting the persistent mode from the storage API.

https://developer.mozilla.org/en-US/docs/Web/API/Storage_API