I've chosen to have plain-text files displayed with thumbnails in Nautilus. However, this has suddenly stopped functioning the other day (after upgrading to Ubuntu 14.04 I guess).
But there's no such problem when I log in to a Guest session, so the problem is probably in user-specific settings, BUT WHERE? (I emptied the thumbnail cache, but this made no difference.)
I wonder what else I can do in my user settings/configuration to fix this with so few options left in the Preferences menu :)