Simple but will it will break all sorts of automation down the line? All the other adlists are txt and I don't know how they would handle other file types, even if the content is unchanged.
At least file extension is limited and externally visible (and thus accountable) to third party behavior, which should limit the worst complexity excesses.
Is filesystem metadata actually different (theoretically) from extension? Or just data in a different format?
Extension seems a nice balance between simplicity / brevity and utility, albeit as a hint, not a commandment.