Very cool. So QImage supports this file format now such that you could open the image in, say, Gwenview?

-1 for making a dedicated thumbnailer.
There's a Qt heif image plug in in the works in Qt. If any, we should help there or at least get our own into kimageformats, so Gwenview and others can open such files properly.

In D27087#604477, @ngraham wrote:

Very cool. So QImage supports this file format now such that you could open the image in, say, Gwenview?

No, as far as I know there is no QImageIOHandler for HEIF files. However, applications such as GIMP, Krita, and digiKam are able to open HEIF files.

In D27087#604479, @broulik wrote:

There's a Qt heif image plug in in the works in Qt. If any, we should help there or at least get our own into kimageformats, so Gwenview and others can open such files properly.

I assume that you want to get the first image stored in a heif file and use it as the thumbnail. I'm not sure whether it's a good idea to completely ignore thumbnails that are associated with the primary image.

Where can we see the status of the HEIF Qt loader? If it correctly implements scaled image loading, then a separate thumbnailer indeed makes not much sense. If, however, it always loads the full image, then scales it down, we got a case for a separate thumbnailer such as this one.

https://codereview.qt-project.org/c/qt/qtimageformats/+/236077