"How would one select by 'extension'?"
Well, if you mean how it would be programmed, sorry, but that's outside my area of expertise. But if you mean 'Can it be done?', the answer is "Yes".
I know from experience using it that Nikon's ViewNX will both select and view by file extension. I gather from the manuals of ACDSee Pro2 and idImager that these programs will also do this, or analogs in their respective ways of doing things.
Your suggestion of sorting by {type} is a helpful one. But you still have to do the sort, then click on the first JPEG, then shift-click on the last one you want, then do whatever you want to with the files. Certainly not a major problem, but being able to select by file type would cut out some steps.
Having said all that, PM is still the best browser/organizer for NEFs. Just suggesting a way it could be even better.