Comment # 3 on bug 1002440 from Wolfgang Bauer

(In reply to Episteme PROMENEUR from comment #2)
> same problem with ffmpegthumbnailer
> 
> it does not appears in dolphin settings then no way to activate it

It is called "ffmpegthumbs" as I wrote.
Install that and it will appear (you need to restart dolphin though).

ffmpegthumbnailer is not even related to KDE at all AFAIK.