Enable thumbnails for AVIF images

Merged Daniel Novomeský requested to merge (removed):master into master

AVIF image format is now supported by kimageformats

This change activates thumbnails in dolphin.

image/avif mimetype is declared in the system via shared-mime-info 2.0. On systems using older versions of shared-mime-info, there is chance that the mimetype was installed via some other packages (for example libheif).

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