Does anyone know a way to do so?
You can see thumbnails via the “RAW Photo Camera Files” preview plugin provided by kdegraphics-thumbnailers.
That’s already installed, and I can view NEF (RAW) files, but not standard DNGs, strangely. Perhaps that specific filetype is unsupported. Worth requesting at bugs.kde.org?
That’s weird, let’s go ahead and file a bug. I only have a limited knowledge of DNG, but I think there’s different “types” of DNG (under the same extension). My iPhone DNGs render fine, so maybe the underlying RAW library needs to gain support for yours?