IMHO the apps should be storage agnostic, with setup including choosing storage (possibly including paid plans).

Meditating that through a standardised storage connector would mean the IoT apps have very little to do.

That could be something like SFTP, but there are some interesting things like https://remotestorage.io/ that allow you to write your own storage provider so decentralised apps can choose their own data store/change storage later without changing the app.