Nextcloud use ffmpeg to generate thumbnails from video files, so you need to install ffmpeg on your server first:
sudo apt install ffmpeg
Now configure nextcloud. Open configuration file /config/config.php and add this lines:
'enable_previews' => true, 'enabledPreviewProviders' => array ( 0 => 'OC\\Preview\\Image', 1 => 'OC\\Preview\\Movie', 2 => 'OC\\Preview\\TXT', ),
That’s all, it is very simple 😉