You could use a forum description, or better even forum rules (displayed on every page of the forum).
There is no way to enforce adding an attachment to a post without some custom coding or an extension. Also you require a zipfile but the contents of than can only be verified after the fact.
Spelling is freeware, which means you can use it for free.
On the other hand, it is not open source, which means you cannot change it or publish it in a modified form.