Exporting e.g. https://en.wikisource.org/wiki/Keeban_(Little,_Brown_and_Company) which has | cover = Keeban cover.jpg results in:
$ ./bin/console a:e -l en -t 'Keeban_(Little,_Brown_and_Company)' In BookProvider.php line 305: [ErrorException] Warning: Undefined array key "imageinfo" Exception trace: at src/BookProvider.php:305 App\BookProvider->getCover() at src/BookProvider.php:112 App\BookProvider->getMetadata() at src/BookProvider.php:48 App\BookProvider->get() at src/BookCreator.php:49
This is because the cover image is actually Keeban_cover.png, but the template outputs <span id="ws-cover">Keeban cover.jpg</span>.