How I solved Dolphin not showing thumbnails for video files

Problem

On KDE Plasma 6 with Dolphin 25.12, video files (.mkv, .mov, .mp4) showed no thumbnail previews in the file manager — just generic icons. The “Configure Previews” dialog listed ffmpegthumbs as enabled, and ffmpeg itself could extract frames from the files without issue. Yet no video thumbnails ever appeared in the thumbnail cache (~/.cache/thumbnails/), while image thumbnails worked fine.

Solution

Two separate settings were blocking video thumbnails. Both need to be fixed.

1. Enable the global preview switches in `kdeglobals`

Dolphin’s per-view “Show Previews” toggle writes to ~/.config/kdeglobals. If these are false, no thumbnail plugin runs at all — regardless of what’s enabled in Dolphin’s plugin list.

~/.config/kdeglobals

[KFileDialog Settings]
Show Inline Previews=true
Show Preview=true

You can also flip these from Dolphin’s UI via View → Previews, but editing the file directly is faster and verifiable.

2. Remove the GStreamer video thumbnailer from the plugin list

This was the real culprit. Dolphin’s PreviewSettings plugin list in ~/.config/dolphinrc had gst-video-thumbnailer listed before ffmpegthumbs:

~/.config/dolphinrc

[PreviewSettings]
Plugins=...,gst-video-thumbnailer,...,ffmpegthumbs,...

KDE tries thumbnail plugins in order. On my system, GStreamer hung trying to decode the H.264-in-Matroska files — gst-launch-1.0 on the same file never returned. When the first matching plugin hangs or fails, Dolphin does not fall through to the next one, so ffmpegthumbs was never given a chance despite being enabled.

Fix: remove gst-video-thumbnailer from the Plugins= line (keep gst-audio-thumbnailer if you want audio previews). The resulting line keeps ffmpegthumbs as the sole video thumbnailer:

~/.config/dolphinrc

[PreviewSettings]
Plugins=appimagethumbnail,audiothumbnail,imagethumbnail,directorythumbnail,ffmpegthumbs,fontthumbnail,jpegthumbnail,rawthumbnail,svgthumbnail,webp-pixbuf

(Trim the list to whatever you actually want enabled — the key change is dropping gst-video-thumbnailer.)

3. Clear the stale thumbnail cache and restart Dolphin

shell

rm -rf ~/.cache/thumbnails
kquitapp6 dolphin
dolphin /path/to/your/videos

After a few seconds, video thumbnails should start appearing in ~/.cache/thumbnails/large/.

How I diagnosed it

The two settings above weren’t obvious — I had to rule out several dead ends first:

Check the plugin is actually installed and loads its dependencies:

shell

ls /usr/lib/x86_64-linux-gnu/qt6/plugins/kf6/thumbcreator/ffmpegthumbs.so
ldd /usr/lib/x86_64-linux-gnu/qt6/plugins/kf6/thumbcreator/ffmpegthumbs.so | grep -i 'not found'

The .so existed and linked cleanly against libavcodec.so.62.

Confirm ffmpeg can decode the file at all:
shell
```
ffmpeg -ss 2 -i "video.mkv" -frames:v 1 -f image2 -y /tmp/thumb.png
```
This produced a valid 1080×1920 PNG, so the files and codec libraries were fine.
Inspect the thumbnail cache to see what was being generated:
shell
```
# KDE keys thumbnails by MD5 of the file:// URI
uri="file:///path/to/video.mkv"
hash=$(echo -n "$uri" | md5sum | cut -d' ' -f1)
ls ~/.cache/thumbnails/large/$hash.png
```
The MKV files had no cached thumbnails, while hundreds of JPGs from other directories did — confirming the video pipeline specifically was broken, not thumbnailing in general.
Reproduce the GStreamer hang directly:
shell
```
gst-launch-1.0 filesrc location=video.mkv ! decodebin ! videoconvert ! pngenc ! filesink location=/tmp/gst.png
```
This command hung indefinitely and never wrote /tmp/gst.png. That matched the symptom exactly: the first-listed video thumbnailer stalls, and Dolphin never tries ffmpegthumbs.