Finder not displaying a folder icon. How do I get it back?
I found some useful info on this Apple support discussion:


  First verify from Finder>View>Show View Option>Show icon Preview
  
  
  
  If that is check, toggle it off and on.
  
  If no success
  
  
  Open to the Preferences folder of  your home directory, or from  Finder>Go>Go to Folder, copy &amp;  paste: ~/Library/Preferences/
  com.apple.finder.plist
  Delete the com.apple.finder.plist
  From the Finder icon on the  Dock, hold the control option key  and click the Finder icon>select 
  Relaunch.
  


If that doesn’t work try updating our Dropbox.

I found someone with a similar problem online:


  I contacted support via chat, and they helped me understand that the dropbox version was outdated. now updated and it's working. thanks!



https://www.dropboxforum.com/t5/Installation-and-desktop-app/MacOs-High-Sierra-10-13-6-after-update-icons-missing-in-finder/m-p/319945#M58452