I already know that Pages can add .doc files too, but my knowledge ends at that. Does anyone have an exhaustive list of files that can be added to Documents in the Cloud through Apple apps (Preview, iWork, TextEdit, etc) only?
If you consider the Finder to be an Apple app, the answer is any arbitrary file.
From my experimentation, it appears files dropped into folders in the
~/Library/Mobile Documents hierarchy get synched to other computers using the same iCloud account, regardless of type.