How to Fix Generic File Icons in macOS Catalina?
Rebuild the LaunchServices Cache using this terminal command (Tested and works on macOS Catalina 10.15.2):

sudo /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user


DropBox Note: 
If you have Dropbox running, you may have to: 1) restart the mac. 2) Pause Syncing and wait for a minute or two. 3) Resume syncing. This should force DropBox to rescan the shares and re-attribute the files and folders.