So, MongoDB with a file URL? (Hint: that's you could implement the MVP of this, and if you use a URL you can even reference user files they don't store locally)
And file permissions are dead? Because nobody has kids that use the same desktop they do?
This is (from a technical standpoint) the silliest goddamn thing I've ever heard.
From a product standpoint, you could probably pitch and get a few M. Why the fuck not.