The forum has been updated. Now if you want to post a track and .caustic file it's as easy as filling in two fields.
1. Go to your track page on soundcloud and find the Full url to your track;
2. Next paste the URL into the Soundcloud Track field;
3. Click the Browse button to locate your local .caustic file and hit Upload.
4. Write your forum post Body data as usual and Submit!
5. See the result.
Hopefully this will make your community sharing 100% more fun!
Remember, when you submit a caustic file, you are agreeing to the public domain sharing of your content.