Enable Video Thumbnails in KDE Dolphin and Xfce Thunar

Open Dolphin

In dolphin, enable previews then go to
Settings, Configure Dolphin.
General, Previews
Put a check in Video File

The Thumbnails created by Dolphin will be shown in Thunar too! Works for GoPro Mp4 files.

Src: https://forums.opensuse.org/showthread.php/413260-Get-Dolphin-to-show-thumbnail-preview-frame-of-picture-amp-vid/page2?s=c6d1560afe19c001cc534b9d663d11f4

for manual fiddling: see the config files in /usr/share/thumbnailers – I think those are only used by thunar not dolphin.