Fortunately for you this shouldn't be outdated for a little while longer, since this only covers "origin private" filesystems. Which, in other words, are just a file-structured IndexedDB and not actual local file access.

This is fantastic! How haven't I heard of this before?

Does anyone know if there's a list of small, little-known extensions just like this?

This is awesome. What other web apps are out there that use this API that I've been missing out on?