PDF Thumbnails:
resource.dopus.com/viewtopic.php ... thumbnails
DWG Thumbnails:
I don't know much about the DWG format except that it's something to do with AutoCAD. If AutoCAD (or some other tool) doesn't provide the OS with some kind of thumbnail generator then a plugin would need to be written to generate thumbnails for DWG files. To do that would require some kind of library or suitable documentation for handling the DWG format but I don't know if such things are available.