Is it possible in nautilus that it shows thumbnails only for pictures and for all others like videos, pdfs, etc thumbnails are disabled?
In dconf-tool, navigate to org.gnome.desktop.thumbnailers then in disable key within brackets write mime type of files, it will look like this.
['application/pdf', 'video/x-flv', 'video/mp4', 'video/webm', 'video/3gpp', 'video/x-msvideo', 'video/quicktime', 'video/x-ms-wmv', 'audio/mpeg', 'audio/mp4', 'video/mpeg', 'video/ogg', 'video/x-matroska', 'video/x-ms-wmv']
then delete .thumbnails folder in home folder. DONE
Open Nautilus file manager and click on "Edit," "Preferences" and "Preview" from the Nautilus window that you just opened up. This will take you into the preview preferences section of Nautilus.
Select "2 MB" from the options on the pull-down menu next to "Only for files smaller than:." This will work if most of your images are taken in standard definition and saved in JPEG format. If your regular images are typically in higher resolutions and saved in a non-compressed format, you may need to up this setting a little. The main focus of this step is to make sure that you can preview all of your image files but don't need to see a preview of your much larger video files.
Click "Close" on the preview preferences window, the changes will save automatically. Close Nautilus by clicking the "X" at the top right corner of the Nautilus window and restart Nautilus to view your changes.
Problem Solved ;-)