I found this solution on a forum, but somewhy this forum was not accessible the second time i tried to read it, as it insisted that i registered. It was still avaible trough google cache, but to make sure information is free, i will publish the solution here as well.

So you probably have wanted to play custom maps in Call of Duty 4 without having to run a mod (who usually doesnt have the possibility to make and save classes). A CoD4 custom map-package usually contains three files: a *.iwd file and two *.ff files. Read more »