Event attachments: Adding an attendee to an existing event does not prompt to create a share link

Steps to reproduce

1. Create an event (don't save!)
2. Click "More details"
3. Add an attachment
4. Click "Save"
5. Close the event
6. Open the event again
7. Add an attendee "Bob"
8. Click "Save"
9. Bob receives the invitation, but the attachment link is the internal link. Bob cannot access the file unless he has access to it through other means (e.g. it's a file from a group folder).

Expected behavior

There should be the prompt asking to create a share link.

Actual behaviour

No prompt to create the share link.

Reproduce the issue using the listed event and attachment steps, then inspect the event save flow and the attachment share-link prompt handling. Confirm the behavior when an attendee is added to an existing event, and consider the issue done when Bob receives an accessible share link after saving.