Where do you see the file-icons, in LR’s Import panel or an operating-system dialog when you’re browsing for the files?
I can see an embedded preview thumbnail in LR’s Import panel when I look at the Leica M Monochrom DNG files from links below the last set on this page of samples—they may download as TIFFs but they are DNGs so rename them if necessary:
http://www.focus-numerique.com/test-1634/compact-leica-m-bruit-electro nique-12.html
If you are talking about an OS-level file-browser dialog not showing thumbnails that just means your OS doesn’t have the necessary codec to interpret the raw files.
If you are referring to the LR Import panel then perhaps upload one of your files to www.dropbox.com and post the public download link in a reply, here, for others to look at.