The API documentation says that nova.path.splitext(path)[0] returns “the path + filename without the extension,” but in fact the path is discarded and only the filename (without the extension) is returned.
In order to get what the documentation says, one needs something like this instead:
nova.path.join(nova.path.dirname(path), nova.path.splitext(path)[0])
(or some custom JavaScript, of course).
Thank you!