Applications
└─ Nautilus

Nautilus is a file manager for the GNOME desktop.

Thumbnails

The configuration for thumbnailers is located in

• /usr/share/thumbnailers
• /usr/local/share/thumbnailers
• ~/.local/share/thumbnailers

Check the mime types in the thumbnailer entries to debug thumbnails not being generated.

The mime type of a file can queried via xdg-mime query filetype FILE.

Prominent thumbnailers:

• ffmpegthumbnailer (for audio/video, from the media-video/ffmpegthumbnailer package)
• gdk-pixbuf-thumbnailer (for images, from the x11-libs/gdk-pixbuf package)

Note that Nautilus shows thumbnails for the current device only by default (can be changed in settings).

References:

• Ask Ubuntu: How to create custom thumbnailers for Nautilus, Nemo, and Caja?