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By the default the cover thumbnailer is disabled. The reason is that it sends information (movie title extracted from the filename) to online services to retrieve a poster. This information is normally harmless, but it is private, so therefore it's not enabled by default.
In order to use the cover thumbnailer you need to create a config file to white-list locations. If the thumbnail file is a child of one of the locations, it will try to download a poster.
[General] # a ; separated list of white-listed locations Locations=/home/yourname/movies/;/home/yourname/downloads;sftp://yourhost.com/home/bla; [TheMovieDB] # API key to use themoviedb.org information #API-key=your-private-api-key
The cover thumbnailer supports two different API backends:
The Open Movie Database API by Brian Fritz. The used posters are most of the time retreived from IMDB.
A free and community maintained movie database. This API however required an API key.
After creating the rc file, it is best if you log-off and on again to restart thunar and tumbler, because both cache the mime-type matching. Else the cover thumbnailer won't be used.