file format appendix

[K. Berry]

    It might be a nice idae to flesh out this list and add a more verbose 
    description of what the individual file types are.  If I had more time, 
    I might write up something, but not at the moment.

In the absence of anyone having the time, I'm dropping this
appendix. There's too many cases to deal with. Not all files with the
same extension go in the same place, and we'll end up describing every
extension on every system. And describing only *some* file extensions
seems guaranteed to frustrate and cause complaints.