I cannot find how to allow connection from unprivileged port on NFS. I found someting about the option nfs_reserved_port_only in rc.conf but it's seem to not work on FreeNAS 8.2.
I found a dirty solution :
Modifying line mountd_flags in /etc/defaults/rc.local to add -n switch.
The line looks like after that :
mountd_flags="-nr" # Flags to mountd (if NFS server enabled).
But it might be better to have a checkbox in the web interface