You should probably use the `CoverDownloader` class to determine the filename based on the episode's parent object (=a `PodcastChannel` object) - this way, you will be guaranteed to get the right filename, even if it has a different extension. The `CoverDownloader` has an option to just get the filename, and not download anything (despite its name). See: https://github.com/gpodder/gpodder/blob/master/src/gpodder/coverart.py#L58