Event attachments: Unable to create sharing link when using the file picker

[kesselb]

Steps to reproduce

1. Create an event (don't save!)
2. Click "More details"
3. Click "Add from Files" (it's important to pick a file from picker, don't upload one)
4. Add an attendee
5. Click "Save"
6. A modal is shown to confirm creating the share link
7. Click "Invite"
8. See an error toast "Wrong path, file/folder does not exist"

Expected behavior

It should work 🥳

Actual behaviour

It does not work 😞

There's a POST request for /ocs/v2.php/apps/files_sharing/api/v1/shares with the body {"shareType":3,"path":"/Calendar/welcome.txt"}.

The issue is the static "Calendar" prefix. That only works for newly uploaded files, but not if an existing file (outside the calendar directory) was picked.

Calendar app version

stable5.0