Now, Dropbox is running inside a container using bind mounts, because I don't trust random binaries downloaded from the network having complete and free run of my system. But it is indeed an ext4 file system, despite Dropbox complaining thinking that it isn't. From inside the container:
I understand that running Dropbox inside a docker container is probably not what you would consider "a supported configuration". But I can assure you that in fact the Dropbox folder is using a supported file system (ext4, with xattr's enabled).
P.S. Dropbox isn't really running as root; it's inside a user id namespace as well.
P.P.S. In case you're curious, here's how I'm setting up the Dropbox container: