File associations and MIME types

Let's say for example that I have a .zip file that I want to rename to some
other random extension (I'll now call it a .abc file), and I want windows to
handle .abc files just like .zip files, using zipfldr.dll. How can I do this
in Vista? In XP (and all previous Windows from what I remember), when you
went into the list of File Types (which is now under Default Programs), you
could add a new file extension, then select a MIME type to treat the file as.
All Vista seems to let you do is select a program.

(Please don't respond to this post by saying "Just go into Default
Programs... it's easier now". That's all I've been able to find on this
subject so far.)