Apple tried it...
Apple resource fork tried something alike - but it wasn't portable. The file extension can traverse any file system that can't recognize other ways to store a file metadata. Plus, managing this kind of metadata in a non-ambiguous way would require a central authority to assign file types ID - reading some bytes from a file header is not enough. Think about all the howls from the anti-MS crowd if MS had setup anything like that instead of letting everybody to create their own file types, and register them with Windows.
Biting the hand that feeds IT
