If you want to do things with a Buffalo box, the box was not designed for
by Buffalo, you are out of luck.
They did "nearly" everything to prevent the user to add anything useful by
themselves, without "hacking" the box.
There is no thing like a filemanager build in. You can install a PHP based
solution into the internal web browser as a "simple" workaround, but this
requires knowledge about installation of PHP packages.
Without "hacking" the box, you will also not be able to "update" bittorrent.
According to your own estimation, you should sell the box and buy one,
which is designed to let the user add things.
Remember for the next time: buying cheap means buying twice.
Maybe someone else in this forum is willing to guide you through a hack
of your box, but be aware that this will always be a risky business with
the possibility of a semi-bricked box (read: you need TFTP or even JTAG
to revive it).