I have openSUSE Leap 42.2 with KDE, I have installed ffmpegthumbs set Dolphin to show previews for videos and turn previews on. But only video file type, for which the preview shows is *.mov, for others like *.avi, *.mp4, *.mkv it doesn’t work.
I already solved it myself. The solution was to update ffmpeg packages from the Packman repository.
I do wish out of the box openSUSE could have all the proper codecs installed but unfortunately for legal reasons, that’s just not possible. I am grateful to all those maintainers of the Packman repository for all those packages so that we can install and go so easily.
Glad you were able to work out your multimedia issues!