SUMMARY
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In Dolphin you can on/off the previews. But if you turn the previews on, than not all file types has previews. The reason is simple, you can turn previews on/off per mimetype in the settings.
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STEPS TO REPRODUCE
1. Turn on the file preview
2. You get a mix between files with and without previews.
3. Go to settings and turn ALL previews on. Than you get nearly for all mimetypes previews when you turn the preview on. Exclusive video files, therefore you need additional packages.

EXPECTED RESULT

User turn previews on, should show for all available mimetypes previews.

In the past, the default setting for the preview setting was, turn previews in general off and show them only if the user change the setting. This was cause the previews need hardware resources to generate AND it's faster without previews. Since the last 10 Years the hardware get faster AND if you want to navigate faster, the user can turn off the previews (for all file types).

I would suggest to set the default setting to show preview and if the user don't want a preview, than the user can change the behaviour in the settings. For Network stuff, there is a separate setting already there in dolphin.

What would be awesome, if you have a package not installed for e.g. video previews, it would be awesome to show at the dolphin settings a button with a link to discover (search) where you can than activate the missing previews.