When you attach something in Addon Discussion (be it map/mod/music/gfx, or whatever else), specify the license you are releasing your work under as well.
The license types can be:
- Public Domain or CC-0: Users can do whatever they want with your work.
- CC-BY-3.0 + GPLv2: As above, but users must attribute you when modifying/redistributing your work.
- CC-BY-SA-3.0 + GPLv2: As above, but users must allow modifying/redistributing of anything they create using your work.
Thanks for understanding!