What all is there to the extras, particularly the details of the design documents?

The "design documents" is a 23-page PDF file containing scans of rough notepad sketches and descriptions, mostly level design/structure brainstorming. The hand-written text is a bit hard to read at times, though there are inserted notes pointing out the relevant parts of the game. Also included are two text files: a short work journal and the repository commit log.

Outside of that, there's nine pieces of concept art (mainly low-res), eight 150x150 avatars, three wallpapers in four sizes each (1280x1024, 1600x1200, 1920x1080, 1920x1200), and 54 mp3 tracks of music.