I am experiencing the same problem. It seems that, like bug #1505014, the thumbnail services from KDE Graphics (PDF, PS, RAW, etc.) have not been ported to Qt 5/KDE Frameworks 5. OpenDocument Format (ODF) thumbnails/previews are also not available. I have kdegraphics-thumbnailers 4:15.12.1-0ubuntu1 installed.
I am experiencing the same problem. It seems that, like bug #1505014, the thumbnail services from KDE Graphics (PDF, PS, RAW, etc.) have not been ported to Qt 5/KDE Frameworks 5. OpenDocument Format (ODF) thumbnails/previews are also not available. I have kdegraphics- thumbnailers 4:15.12.1-0ubuntu1 installed.