Somehow I managed it to get thumbnails for flac audio files, which have covers included in their tags, to show up in Ubuntu version 11.10. But since I reinstalled for Ubuntu 12.04, they don't show up anymore and I don't know which package I need to install to re-enable.
I've already installed "ffmpegthumbnailer", "ffmpegthumbs", "kdegraphics-thumbnailers", "kffmpegthumbnailer", "mplayerthumbs" packages. Do I need "tumbler" to get it working again ?
One possible reason: It may be possible that some PPAs I used in 11.10 contained newer version of the thumbnailers than shipped with Ubuntu 12.04. If somebody could point me to the right package I could update it again.