When configuring Port Mapping with the AirPort Extreme (802.11n) base station to share Personal and Windows File Sharing, AirPort Utility may display an exclamation point next to the default public port assignments, indicating an issue.
By default, Personal File Sharing uses port 548 as the public port. Windows File Sharing uses 137-138 as default public ports.
These will need to be changed to an unused port in order to share these services between the base station network and the external network. Also, you should avoid port 548 because it is reserved for AirPort disk.
If the sharing a hard disk function is not needed from your base station, go to the Airport Utility and uncheck the 'Enable file sharing' checkbox in the File Sharing tab. This will free up port 548 and ports 137-138.
See this article for information about which ports are already in use by other services.