It’s now possible to add attachments to events via Nextcloud Calendar interface. But how can I access these attachments in Android calendar apps like aCalendar, Etar etc.? I sync my calendars via DAVx⁵.
That’s not possible at the moment. Extra developments are required on both Nextcloud’s side and CalDAV clients to support this feature.
See Add attachments to events by sazanof · Pull Request #4251 · nextcloud/calendar · GitHub and next comments for details.