Yes, custom file system support is still in beta - it had a lower pripority than HTTPS or elliptic curve cryptography, unfortunately.
I just sent you a link to the current Rebex File Server with IFilesystem support based on the latest release - 2017 R1. However, please note that this is part of Rebex.FileServer.dll (Rebex.FileTransfer.dll is FTP/SSL and SFTP client).