- QSettings object needs special care, as file was re-created on exit
- Append a cleanup function as soon as possible to ensure temporary dirs & files are removed in any case
- prefer addCleanup() over tearDown(), keeping creation and cleanup together
That can happen if a new cover art type is added in MB that Picard doesn't yet
know about. Instead of raising a KeyError (and crashing), just return the raw
type, which is good enough until we have something better to show.