It would serve as a key, defining data types and encapsulating algorythms for decyphering those files. Jeff Rothenberg, in an article for Scientific American, suggests encapsulating software with the stored digital media as a way to get at the media through time. MacCarn proposes the use of platform-independent algorythms to decode file types.
The Rosetta stone might state in effect, "This system uses MaRC, which is defined as such-as-such" or "This system was originally recorded on 422 Video, which is defined as so-and-so." In addition, the Rosetta stone might include some form of mapping among among multimedia file formats or even classification or cataloging systems. The Rosetta stone would also serve as registry for unique identifiers.