If you stick with PC-to-PC scenarios, then keying off the
.EXT should work quite nicely. NeXT had a lot of success with their
browser using .extensions even in the Unix (Mach, actually) world.
I'd rather see this than a plethora of MIME types.
I might agree with you in the restricted case of PC to PC communications. The
real world is not so limited, however.
For example, what does an extension like .DOC mean? You'll find that it had
very different meanings on VMS, UNIX, DEC-10, DEC-20 and PC systems. DEC-10's
often use .DOC for documentation files, and CUSP's like RUNOFF make specific
assumptions about this usage. I'm not sure this carries over to DEC-20 though.
On VMS .DOC files are what DECPresent produces. The internal structure of these
.DOC files is DDIF (an instance of ASN.1 and BER that's close to ODA,
actually). And on UNIX .DOC has no special meaning as far as I know.
There are lots of other overloaded extensions that are inevitably going to
cause similar problems. And there are systems which simply don't have the
concept of file extensions (Macintoshes). MIME types, on the other hand, are
precisely and uniquely defined. As such, they provide critical information that
file names will never be able to match.