Dolphin under Gnome doesn't display any thumbnail, how to fix?

Question

I have installed the Dolphin file manager under Ubuntu 18.04/Gnome (with apt install). It doesn't show thumbnails for any file, neither in the folder nor in the information panel (to the right). See screenshot. How can I fix it?

Also, in the Dolphin Preferences I noticed that under General > Previews > Show previews for: there is a blank list, which I am not allowed to edit. I believe I should have a list of file types there. See screenshot.

I have tried deleting the content of ~/.cache/icon-cache.kcache and ~/.cache/thumbnails.tar, then logging out and in again, but no change.

I also tried the fix that should address video clip thumbnails, but got an error in the process:

-> sudo apt install ffmpegthumbs mplayerthumbs kffmpegthumbnailer
[sudo] password for fanta: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package mplayerthumbs is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'mplayerthumbs' has no installation candidate

Note the issue is not limited to thumbnails of video clips, it applies to all files, including images (Jpeg, PNG, etc.).

Update: I have tried adding the KDE Plasma repository

deb http://archive.neon.kde.org/user bionic main

then run a sudo apt upgrade and re-booted. I didn't actually install KDE Plasma. Now the thumbnails are working, but I am getting other malfunctions, including the Software & Updates app that doesn't start (software-properties-gtk issues "aptsources.distro.NoDistroTemplateException: Error: could not find a distribution template for neon/bionic"). Therefore I am undoing the changes.

Answer 1

In Dolphin does not show thumbnails from 2014, the following additional packages are recommended:

• ffmpegthumbs: video thumbnail generator using ffmpeg
• mplayerthumbs: video thumbnail generator using mplayer (no longer needed and not in the bionic repos)
• kffmpegthumbnailer: video thumbnailer for KDE (doesn't appear necessary in KDE Plasma5)

A more recent answer from 2016 there indicates that kio-extras is also needed.

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I had recently installed dolphin on Ubuntu 18.04 and could view thumbnails and also had the Show Previews pane that was missing on your system.

I looked through the appropriate history.log file in /var/log/apt to see what thumbnail-related packages were installed along with dolphin. I found two:

• ffmpegthumbs and
• kdegraphics-thumbnailers

Indeed, these were the only two packages with "thumb" in their names on my system:

$ apt list --installed | grep -i thumb

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

ffmpegthumbs/bionic,now 4:17.12.3-0ubuntu1 amd64 [installed,automatic]
kdegraphics-thumbnailers/bionic,now 4:17.12.3-0ubuntu1 amd64 [installed,automatic]
$ 

kio-extras, the package recommended by the second answer, along with kio-extras-data, was installed on my system, not along with dolphin but when I installed gwenview, Kubuntu's image viewer.

Anyway, this is what dolphin looks like:

Dolphin showing thumbnails and the pane to select which previews are shown

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Edit:

kio-extras is a "recommends" for dolphin in Kubuntu 19.04.