Gwenview should support OpenRaster files. Many years ago, KDELibs got support (https://bugs.kde.org/show_bug.cgi?id=290528#c5), and as a result, KIO and hence Dolphin are able to display thumbnails for .ora files. But Gwenview can't actually open them. This file format has a FreeDesktop spec: https://freedesktop.org/wiki/Specifications/OpenRaster/
I no longer care about this, and due to the lack of CCs or duplicate bug reports, apparently nobody else does, either.