To proceed, just copy /resources/config/mimetypemapping.dist.json to /config/mimetypemapping.json (in the config/ folder at Nextcloud’s root directory; the file should be stored next to the config.php file). Afterwards add the two following lines just after the “_comment” lines.
That would do in a setup not using docker. In case you you use the Nextcloud app in UCS you will find the original file inside the container
# univention-app shell nextcloud ls /var/www/html/resources/config/
ca-bundle.crt
mimetypealiases.dist.json
mimetypemapping.dist.json
I’d also have a look at the discussions here regarding the persistent changes of the config.php for Nextcloud (hint: /var/lib/univention-appcenter/apps/nextcloud/data/integration/config).