Rather than a hardcoded list of only two thumbnailers.
All of this is done under the hood so the QML part doesn't need to be changed.
This is the same behavior as Dolphin uses, when the list of thumbnailers is empty, the default set (which includes all thumbnailers except textthumbs) is used.

Before

• Verified that all the thumbnails are shown
• Verified that the previewer config dialog has the correct plugins checked
• Verified that changing the previewer config updates the thumbnails and has the setting stored, execpt when the default set of previews is enabled, nothing is stored so that when defaultPlugins would change, it is picked up automatically

I don't exactly know why the ttf thumbnail looks different from Dolphin where it is just a couple of letters instead of a full sentence