FTPony and FTPbrony are both FTP servers, not FTP client.
FTP server on 3DS = you need a PC to connect to the 3DS and browse the 3DS SD Card content.
FTP client on 3DS = You use the 3DS to connect to FTP servers (local PC or internet) and browse files on the distant server.
He asked if a FTP client homebrew exists, but no. Nobody coded that kind of homebrew yet. (even on other consoles, there are only servers, no client)
I don't know how useful it could be. Maybe access PC files using the 3DS from your bed? could be useful to backup savegames to PC directly from the 3DS (like a cloud service, when you are not at home), without going to the PC to access the 3DS.