By default ASA will do FTP magic to let data pass trough the device correctlly (active and passive). By default this is only active on port 21, but if your FTP server runs on another port you should teach ASA this. Full info here.

access-list ftp-list extended permit tcp any any eq 2020
class-map ftp-class
  match access-list ftp-list
policy-map global_policy
  class ftp-class
   inspect ftp

That's all.