why won't .cdr files preview as a thumbnail?

I had this problem in Windows 10 and it was as simple as going into the Windows settings, then System then default apps and then scrolling to the bottom of the list and clicking the restore to Microsoft recommended defaults. If that doesn't work for you, this method and some others are explained at the link below.

http://community.coreldraw.com/collaborate/w/faq/133.thumbnails-not-showing-in-windows-explorer-after-installing-cdgs-x6x7