When you use Gnome Shell under Ubuntu or Debian, all LibreOffice files are displayed thru some generic icons in Nautilus.
LibreOffice files are either a ZIP archives embedding a thumbnail of the first document page(.odt, .odx, ...) or a flat XML file (.fodt, .fodx, ...). With both type of documents, it is possible to use or to generate this first page thumbnail as a Nautilus thumbnail.
To make it even more visual, the thumbnail can be outdrawn by the document type official icon. You will then get a display much more visual.
This article explains all the steps needed to configure Nautilus to display thumbnails of LibreOffice documents including document type icon and first page content, providing a simple and efficient LibreOffice documents preview. It is using some very basic tools.
This procedure has been tested under Ubuntu 18.10, Ubuntu 18.04 LTS and Ubuntu Gnome 16.04 LTS. It should be applicable to many Gnome based distributions.