Tumbler has since version 0.1.27 a configuration file to control the priority and locations, file sizes a plugin is used for: tumbler.rc.

To override the default configuration, located in one of the xdg directories on your system, you can copy the rc-file to your $XDG_CONFIG_HOME directory.

NB: $XDG_CONFIG_HOME defaults to ~/.config.

mkdir -p $XDG_CONFIG_HOME/tumbler
cp /etc/xdg/tumbler/tumbler.rc $XDG_CONFIG_HOME/tumbler/

Each group in the plugin is responsible for the settings of a particular thumbnail provides plugin. The following keys can be used to adjust a plugin:

Disabled

Disable the plugin, the values true and false are accepted.

Priority

Numeric value to control the priority of the plugin. If 2 plugins can provide thumbnails for the same scheme + mime-type combination, the plugin with the highest priority is used first.

Locations

A ;-separated path list the plugin will be used for. If the source file is not a child of one of the locations, the plugin won't be used and another plugin with a lower priority will be tried. Absolute paths, environment variables, ~/ and ~username/ are allowed. Leave empty to allow all locations.

MaxFileSize

Maximum size of the source file the plugin will still try to generate a plugin for. The size is in bytes, 0 disabled the check.

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 copy the tumbler.rc file to your home directory like described above and set the Disabled key to false. It is also advised to set the Locations path to the directory where you store your movies.

[CoverThumbnailer]
Disabled=false
Priority=3
Locations=~/movies
MaxFileSize=0
#APIKey=your-api-key-from-themoviedb.org

The movie thumbnailer supports 2 different movie API's. By default the The Open Movie Database API is used. At your choice you can also use The Movie DB, but this site requires an API key, which you have to request or uh… search.
Advantage of the latter is that their database has better support for internationalized movie names.

After creating the rc file, it is best if you log-off and on again to restart Thunar and Tumbler, because both applications cache the mime-type combinations.