There seems to be no way to exclude folders or files from image backups but include in file backups without separate partitions.
The way described here excludes from both and also seems to only exclude files from images which are bigger than roughly 10mb. This is a bit confusing because it may seem that a folder was saved while there are missing all the bigger files.
I would like my images to become as small as possible to make use of saving the same files of different clients only once.
This would also help to get complete (image & file) backups of remote clients by just carrying the large files to the backup server as described here: https://www.urbackup.org/import-seed-remote-client.html