At least someone will be able to store the full specification of the format, or a reference implemetation, along the file that is going to be preserved. It's nothing more, in fact, than one document that refers to another in that the only way to get all the information is to decode both.

You can't store the full spec of a format that's not fully documentes nor keep a runnable reference implementation that has only a binary executable (unless you also preserve the computer it runs on). But then you have turtles all the way down.