Since latest Nautilus 3.26.4, most external thumbnailers are not working anymore under Ubuntu 18.04 or 18.10.
Why ? Because, since few months, some Nautilus external thumbnailers vulnerabilities have come to light (gnome-exe-thumbnailer for example).
As a result, Gnome Shell team has decided to harden thumbnails generation when using external thumbnailers.
First hardening steps have come with Nautilus 3.26.4, where all external thumbnailers are not called directly but thru bwrap, the bubblewrap project sandboxing tool.
Default bwrap options used by Nautilus appear to work fine under Fedora, but fail badly under Ubuntu and Debian based distros.
This short article explains how to reactivate Nautilus external thumbnails generation under Ubuntu Bionic 18.04 and Cosmic 18.10. You should get back thumbnails generation as they were working with previous versions of Nautilus.