If a poster is uploaded to an event, and that poster is a PDF file, an image thumbnail of the first page of the PDF is not shown on the event page. Instead "No poster available. View all artwork" is shown. The thumbnail, however, is viewable in the event art tab of the event page as well as on the MBz homepage of newly added events.

Example: https://musicbrainz.org/event/46de7260-00f7-425f-af28-778bcce044f3

This is due to PDFs being excluded from the index_listing view we use to calculate the frontiest image, which was done back when PDFs did not get thumbnails. Since they do now, we should remove that restriction from the view.