bograt,
Sorry, I did not understand what you want. Do you want to configure the previewer application?
If kaffeine is in the path, just type "kaffeine" on the Preferences->Directories->Video Player.
If kaffeine is not on the path, you must find where it is, it is usually in /usr/bin/kaffeine, so if you installed correctly it should be in the path.
With the right button, you can click on a file name in the Transfers tab and if the first chunk of the file has already been downloaded, you will be able to preview it. Otherwise, the preview will be disabled.
If that does not answer your question, forgive me and please rephrase it, english is not my native language.

Cheers!