The new webp image standard is used a lot this days. Somehow Nautilius is not generating thumbnails for this format automatically.
To add support for this just do the following:
STEP 1: Add support for the new image format.
Create a new file /usr/share/thumbnailers/webp.thumbnailer with the following content.
[Thumbnailer Entry] TryExec=/usr/bin/totem-video-thumbnailer Exec=/usr/bin/totem-video-thumbnailer -s %s %u %o MimeType=image/webp;image/x-webp;
Also make sure totem player is installed, because we are using the totem thumbnailer to generate the preview images.
STEP 2: Restart Nautilius
$ nautilus -q && nautilus &