Malte,
Solution Grove added something like this for the LNW Program.
The URL below should take you to a search page which makes use of categories tied to a file storage instance where you can search files. This makes use of a modified category widget as well that's why the display is like that.
http://www.lnwprogram.org/library/search-categories
Actual categorization was added on the upload page and on the URL creation/edit page.
If you try clicking on a file or selecting multiple files on the result page, then you'll see their categorization. This was client requirement so it was made that way but I guess it should be on the file details page for the stock code. I initially did the code but it has been touched by DaveB and Hamilton as well so I'm not sure how heavily customized it is now.
