On save, I noticed that the image's extension was .jpeg rather than what I thought was the usual .jpg. As both of my other Macs save with the .jpg extension, I figured something was messed up on the iMac. So I (of course) tweeted about the issue. A while later, Shawn King replied with this seemingly odd suggestion:
— Shawn King (@ShawnKing) March 19, 2017
So I tried it, and sure enough, changing the screen capture file format via defaults write com.apple.screencapture type jpg and then restarting the SystemUIServer with killall SystemUIServer changed my default JPEG extension in every app to .jpg.
What's really strange is that I then switched the screenshot format back to png, and the .jpg extension remained. I even went so far as to delete the pref (defaults delete com.apple.screencapture), and still, the extension remains .jpg. So whatever change occurred when switching the default screenshot format, it appears to be permanent.
I tried the same trick for the .tiff extension (which I rarely use, so it doesn't bother me as much), and it sort of worked: Captured screenshots got a .tif extension, but images saved from apps still got the four-letter .tiff extension. Weird.
If anyone knows exactly what's going on with the .jpeg vs. .jpg extension, I'd love to hear the explanation.