Apparently there isn't much system-wide preview support for formats such as XCF and ORA. It used to be better at some point, at least when KDE was mostly QT4, at some point I had fancy thumbnail previews of everything. Everything! Gwenview apparently was also at some point able to preview ORA and maybe the others, but I've never seen that.
So, unfortunately I'm not a programmer or anything, so I can only look for poor man's solutions like a script that converts such files into more...