The ? gets replaced with empty .That is correct .As the gcode is not fdm the thumbnail is empty. As I have plenty of gcode for cnc ,it becomes difficult based on the filename to find the right cnc file. If I need to open the 3d file ,to preview it ,there is no gain for me.The thumbnail makes is very easy to select the correct file.
Is it a lot of work to generate a preview for non fdm g-code? I think other CNC users will be intrested as well.
I can send you a file as example but I do not see a link option here.
Another option is that we have kind of setting as what to use as priority for the thumbnails.Then we can use an image as first priority for the thumbnails.