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.