In article <199712030609.RAA25537@plum.cyber.com.au> you write:
> 
> useful to have some generic properties/flags, such as "this type can only
> be present once" for associating an executeable with a data file.  I'd

Assuming that automatic execution of program on data files is something
that should be handled at the exec() level (and that's a huge
assumption), this is still the wrong way to do it.

Why should typing "foo.gif" at a shell prompt display it, but then when
I copy it to /a (msdosfs), running foo.gif tells me "bad magic"?

If you want to do this, it'd be much better implemented with something
like the EXEC_GENERIC mechanism proposals discusses a month or two back
(look in tge current-users archives). That was inteneded for things like
automatically executing things that really are programs, but which need
to be run under an interpreter (like java bytecode). As was discussed at
the time, there's no reason it couldn't be (ab)used to run image viewers
on gifs or whatever.

Anyway, in reference to all this EIII stuff, you might want to look at
what BeOS has done. I mean, if you're going to do it, you might as well
do it all the way.