There is no support for FTP/SFTP in FreedomBox via App right now. It as an old, simple and effective protocol with a wide range of support for different clients. Most Linux distributions support it already out of the box through there file manager. So why not including FTP/SFTP in FreedomBox?
SFTP should work as long as the SSH server is enabled.
Yes, as you mentioned in another post:
However, that is not a nice experience for the user. There should be a web application, where you can enable SFTP and restrict it to a specific folder. The management should happen via web application.