nixos/nextcloud: add bcmap, mp4 and webm in nginx configuration

This is used by the `firstrunwizard` and has been added in the nginx
configuration documentation of the latest
manual (https://github.com/nextcloud/documentation/blob/cda627b7c8896f3bac8bfde420c0c8b5d1806190/admin_manual/installation/nginx.rst).