Okular can't open *.dds files, used for textures in many games. Clicking a *.dds file in Dolphin opens Gwenview, but it prints thw error "Gwenview cannot display documents of type image/x-dds.".
Do you have kimageformats and qt5-imageformats packages installed? Please ask in a forum of your distribution for exact package names.
Yes, both. The description for qt5-imageformats Arch package says it adds support for TIFF, MNG, TGA, WBMP. kimageformats lists several formats, dds not included. https://api.kde.org/frameworks/kimageformats/html/index.html
More ideas: - check /usr/share/kservices5/qimageioplugins/ - try adding image/x-dds to the MimeType list in /usr/share/applications/org.kde.gwenview.desktop According to http://doc.qt.io/qt-5/qtimageformats-index.html DDS should be supported by Qt. The DDS loader was removed from kimageformats because of the Qt support, but you still need to have the kimageformats installed so that the .desktop file gets installed. See https://cgit.kde.org/kimageformats.git/commit/?id=e5fce91de6ef72f15c7d31b16b2494f534c7157c
Digging deeper, I found https://bugreports.qt.io/browse/QTBUG-59955 If you cannot compile Qt from source, I suggest to ask your distribution to re-enable DDS support in Qt (at the risk of getting bug reports about it).