I have this in my config.php , and i'm getting mp4 previews.
Btw if you want to have thumbnails on new uploads, you need to add a cronjob generating them.
https://github.com/rullzer/previewgenerator#how-does-the-app-work
'enabledPreviewProviders' =>
array (
0 => 'OC\\Preview\\Image',
1 => 'OC\\Preview\\MP3',
2 => 'OC\\Preview\\TXT',
3 => 'OC\\Preview\\PDF',
4 => 'OC\\Preview\\Movie',
5 => 'OC\\Preview\\Photoshop',
6 => 'OC\\Preview\\TIFF',
7 => 'OC\\Preview\\SVG',
8 => 'OC\\Preview\\OpenDocument',
),