NextCloud Files_Anti-Virus Simple Install
Make sure ClamAV is installed and running on your server.
Move to the apps directory in your NextCloud installation
'cd [path to nextcloud]/apps`
Download the app from GitHub. Use:
https://github.com/nextcloud/files_antivirus/archive/stable10.zip for Nextcloud 10
https://github.com/nextcloud/files_antivirus/archive/old-master.zip for version 11
https://github.com/nextcloud/files_antivirus/archive/stable12.zip for Nextcloud 12
https://github.com/nextcloud/files_antivirus/archive/master.zip for Nextcloud 13
[sudo] wget https://[link to the version you need]
Unzip the file into your nextcloud/apps directory.
[sudo] unzip
Chown that folder to your web server user (www-data in my case). (Not sure this is necessary but it keeps things tidy.)
[sudo] chown -R www-data:www-data files_antivirus-master
Rename the directory
[sudo] mv files_antivirus-master files_antivirus
(Not sure if this is actually necessary, but I reloaded my apache vhosts here.)
[sudo] service apache2 reload
Log on to NextCloud and goto Apps. In the “Not enabled” section, enable “Antivirus App for files”
Move to the admin section and setup as necessary.
Done!
edit: updated links in body