From memory and FWIW:
This seems to happen to people who upgraded from Mavericks to EC. The reason seems to be that either the “lsd” folder in
/private/var/db/
is missing, or you may have a corrupted Launch Services database.
To reset the database:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user ; killall Dock
Restart the mac.
This launch services stuff gets cached at startup, so debugging trough changing the plist with a text editor won’t work, unless you restart every time.