This article only mentions in passing the best approach so far: Uniform Type Identifiers (1). When it comes to file types and associations Mac OS X hit a homerun. This mechanism way more powerful than MIME types because:
UTIs use a reverse-DNS naming structure. UTIs support multiple inheritance (which is a good thing in this context) Using UTIs allow per file exceptions.
It really is a shame this was not implemented by any other OS than Mac OS X. It is one of the brilliant technical features it has. Together with bundles (2), they make for very powerful and flexible filing.