Linux Mint 18 KDE Audacity displays wrong extension on save

This has been pretty consistently the case for the last few versions/dot-upgrades of the app.
I have MP3 set in my prefs, but the app always ends the filename of the file I was editing as “.aiff.”
Is there any way to change this?

You mean that Audacity gives the default wrong extension on Export (“Save” is for saving Projects, not audio files).
Yes that is correct - it’s a bug. I believe that it has been at least “mostly” fixed in the up and coming Audacity 2.2.0 version (I’m not sure off hand if it is completely fixed - please raise the issue again if you find the problem in 2.2.0 when it becomes available).