Setting a format does not automatically mean making a wav file.
Instead, check for the extension on the output file name and when we
can't find any good containet, use the same as the input file.
This makes more sense than doing `spa-resample test.flac -r 44100 test.wav`
and ending up with a FLAC file named test.wav. Or
`spa-resample test.flac -r 44100 -f s16 test2.flac` and ending up with a
wav file named test2.flac.
5e6b77cf1e