you must forward 60100-60200, too. As red.adair explained the FG cannot know that your FTP-Server told the client to connect to say port 60101 because the session-helper does not see the server' s response. I' d probably use a static mapping (full IP forward) for a FTP server.
Also make sure to use Passive-FTP on the client side. Otherwise you' ll have to allow IIRC 989 -> Any outbound.