As of !7555, all our GNOME UIs (GNOME, GNOME Mobile, Phosh) come with gst-thumbnailers by default instead of ffmpegthumbnailer. We previously used ffmpegthumbnailer due to it having certain advantages over totem-video-thumbnailer, but now upstream GNOME is considering switching to a new GStreamer-based program. Given that it seems very likely this will go through we have decided to give switching the go-ahead downstream in postmarketOS too to ensure we continue to stay close to upstream.
Unfortunately, due to limitations in pmbootstrap and apk, the old thumbnailer program will remain installed even after the new one has been added to the system. While keeping both installed at the same time shouldn't cause any problems, you are welcome to remove the old one like so:
sudo apk del ffmpegthumbnailer
This is not necessary on new installations as they only come with gst-thumbnailers.