3.5.0: Export Parts to WaveAudio gives .pdf instead of .wav

I'm trying to Export Parts from a score as separate instrument tracks (files) in .wav format. No matter what audio format I select, I always get .pdf files, one for each part (as should be). Has anyone else had this problem? (Export to .wav in 2.3.2 works OK - except of course there's no single-note dynamics.)

While I'm at it, is there any way to modify the synthesizer parameters, specifically to change the sampling rate from 44100 to 48000? That would save me a lot of work in Audacity to resample the files one at a time - although I could probably do them all at once in ffmpeg if I could figure out the right command.