I don't see why you disqualify SSH here. If you want a system that only deals with filesystem operations, I suggest NFS. It is neither firewall-, NAT-, proxy- or browser-friendly, is not cross-platform (Windows support only if you pay extra) and is neither encrypted nor authenticated. But hey, it deals with filesystem operations only.
If, on the other hand, you want a system that meets your last 3 requirements, just use SSH/SFTP.