There some issues with nautilus file browser in 18.04. For starters no clear way to run scripts, no app-runner for instance.
Here is how to install nautilus-actions in 18.04
Here is a fix for lack of contextual menus in 16.04, it should work after you install the above.
And here is a fix for the lost or broken icons:
Since latest Nautilus 3.26.4, most external thumbnailers are not working anymore under Ubuntu 18.04 or 18.10.
I realized the problem when my LibreOffice and MsOffice thumbnailers stopped to work.
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.