Using FTP as the cloud choice for attachments is not recommended, since setting up a properly functioning FTP server and firewall rules are complicated, error prone and hard to diagnose. But it is an option if you need or want it. Once you select FTP in the wizard you'll see the following window:
You can use the "Test FTP Settings" button to test whether the entered credentials and settings are correct. Please note that this only validates the given log in credentials against the given server as existing and able to log in. It's possible to have policies on an FTP server that will allow log in, but refuse uploads for specific users or similar. Setting up an FTP server correctly requires coordinating with a competent IT staff.