Audacity opens WebM files perfectly, but throws the following error:
‘Could not open file X: Incorrect filetype.’
Responding ‘OK’ to the dialog or escaping from it allows the file to be opened or imported normally. You can also bypass the error by simply renaming the *.webm file to *.mkv before opening.
I would suggest adding ‘webm’ to the acceptable list of formats or at least changing the error message to let the user know that the file will be opened correctly. Should be an easy fix to smooth out the user experience.