toPickedMapFile

Wrap a picked content:// as a PickedMapFile for the layer store.

The store never sees a Uri: resolving one is exactly the platform knowledge it exists to avoid, so the read is handed over as a lambda instead. The MIME type is the fallback for the extension rather than being used only when the display name is absent — a picked file with no dot in its name used to resolve to no type and be rejected.