Source: ownyourbits.com/2019/06/29/understanding-and-improving-nextcloud-previews/
./occ config:app:set previewgenerator squareSizes --value="32 256"
./occ config:app:set previewgenerator widthSizes --value="256 384"
./occ config:app:set previewgenerator heightSizes --value="256"
./occ config:system:set preview_max_x --value 2048
./occ config:system:set preview_max_y --value 2048
./occ config:system:set jpeg_quality --value 60
./occ config:app:set preview jpeg_quality --value="60"