Looks like I do actually have to list all types.
https://www.reddit.com/r/NextCloud/comments/rwu0sh/heic_preview_support/
Solved.
Fixed config ends with:
'maintenance' => false,
'enable_previews' => true,
'enabledPreviewProviders' =>
array (
'OC\Preview\PNG',
'OC\Preview\JPEG',
'OC\Preview\GIF',
'OC\Preview\BMP',
'OC\Preview\XBitmap',
'OC\Preview\MP3',
'OC\Preview\TXT',
'OC\Preview\MarkDown',
'OC\Preview\OpenDocument',
'OC\Preview\Krita',
'OC\Preview\HEIC',
),
);