In this Community a find a lot of diskussion about thumbnail in windows explorer. I open a new one, because nothing helped me even though I try everything. I think, my problem is with microsoft onedrive, with i work with. When the skp file is open and I work in, the thumbnail will be shown. After I close the file, it takes about 1 - 2 minutes (backup time) and the thumbnail change to icon. When I go online in my onedrive account, the describtion is “for this file, there seems to be no preview”. What can I do??
I try it with varius files sizes. Even big photoshop files I can see 
I got it!! Go settings in onedrive. Uncheck “files on demand”. Now I can see thumbnails:-)