Add these lines to the ./nextcloud/config/config.php these lines:
'enable_previews' => true,
'enabledPreviewProviders' =>
array(
0 => 'OC\\Preview\\TXT',
1 => 'OC\\Preview\\MarkDown',
2 => 'OC\\Preview\\OpenDocument',
3 => 'OC\\Preview\\PDF',
4 => 'OC\\Preview\\MSOffice2003',
5 => 'OC\\Preview\\MSOfficeDoc',
6 => 'OC\\Preview\\Image',
7 => 'OC\\Preview\\Photoshop',
8 => 'OC\\Preview\\TIFF',
9 => 'OC\\Preview\\SVG',
10 => 'OC\\Preview\\Font',
11 => 'OC\\Preview\\MP3',
12 => 'OC\\Preview\\Movie',
13 => 'OC\\Preview\\MKV',
14 => 'OC\\Preview\\MP4',
15 => 'OC\\Preview\\AVI',
),