Well, if you can hook up a drive to it, and setup ftp, well, you got an ftp file server. It might not be fast, but yeah, it will do ftp. I would personlly try for ssh, more secure, and doing a scp command in terminal will give speeder transfers than sftp.
SSH stands for Secure Shell which is a protocol that allows encrypted communications over networks such as the internet. You set up a SSH server, open up the default port 25 on your router, and use OS X terminal to login to your server's IP address. Once logged in it will allow you to operate as though you were sitting at that computer's terminal input. You can execute operations and access files. It is all command prompt, though.
SCP (secure copy protocol) allows you to transfer files from an SSH server to your local machine much like an ftp program. The files transferred will be encrypted by the protocol as well, so you will be much more secure than standard ftp as well. This can be done in the terminal as well.
that's about it.
Oh, if you need windows access as well, putty is by far the best SSH client IMHO and WinSCP is great as well.