Oh, stupid me !
The solution for my problem was, that used I used the folder name instead the codename for the MediaFileDatasources Libraryname - property.
So the files are listed now.
One last thing I like to ask for (on this topic) is a link to a page that explains how I can influence the way the librarys content is displayed. In only found this small info
on the webparts propertys.
Thanks a lot for your patience (and help of course).
Have a nice start in your working week