In Ubuntu 15.04 (new installation, no upgrade) I cannot find an option to enable thumbnails for text files. In previous Ubuntu versions the first lines of a text file can be seen. Now in Ubuntu 15.04 all text files have the same icon. Also I cannot distinguish empty text files.
As it seems there is no option to enable thumbnails for text files any more in the settings of nautilus.
$ gsettings get com.canonical.unity-greeter icon-theme-name 'ubuntu-mono-dark'