This is closer to a shower thought, given I don’t really know much about how ActivityPub works. Has anyone, or is it even possible to, made an instance designed to work as say a FUSE file system?
I was mostly just thinking about how hard UIs are to make and thought “what if the content was the UI”.
No, the latter is done via a DHT, while the former is a layer on top of IPFS (Filecoin, I guess, as verdare said). Default IPFS behaviour is that you just cache whatever you download, and serve it to others if they request the same file from the network. Basically a huge bittorrent kinda thing. You can also explicitly “pin” files, keeping them cached indefinitely, which is the closest thing to hosting the file, and that’s what Filecoin incentivizes, but people also do it without Filecoin involved.