Comment # 1 on bug 1002440 from Wolfgang Bauer

kffmpegthumbnailer is an old KDE4 based thumbnailer and will only work in KDE4
applications like Konqueror or dolphin4.

For the standard dolphin, install ffmpegthumbs that's included in 42.1 (unlike
kffmpegthumbnailer).

And to really see thumbnails, the installed ffmpeg needs to support the
corresponding codecs of course, so you may have to install it from Packman.