This seems like the most basic thing, maybe that's why I can't come up with the right search terms. I've implemented this for apps past, but those were in previous versions of Xcode (then PBuilder probably), when there were some fields to fill out under "Target." Now those fields are nowhere to be found...anyway, thanks in advance.
Oh, cocoa project, Obj-c.
EDIT: Ok, just found it. You select your app under Targets and hit cmd-i, then add document types. I think it was much clearer before, and I don't see what advantage there is in this new obfuscation; it's not like the rest of the Xcode window is doing any good in this case.