The Paragon Wiki article on costume files is marked as incomplete, but I think it might be complete? I don't know enough about costume files to know, so hopefully the demo editors can edit the article if necessary:
http://paragonwiki.com/wiki/Costume_File If it is complete, please remove the { { incomplete } } tag from the article. If you don't want to edit the wiki, put notes in this thread.
I put this thread in Demo Recording since I assumed that this is where people with knowledge of costume files will likely be.