Document attachments in API v 1.1/(Deck 1?3+)

Describe the bug
Starting with Deck 1.3.0, the attachments property of a card is an empty array. The Deck Android app is no longer able to display attachments.

To Reproduce
Steps to reproduce the behavior:

1. Install Deck 1.3.0
2. Install Deck Android 1.14.6 (or any other version)
3. Add a new attachment via Web UI
4. Do not see it the attachment

Expected behavior
Obvious, API should deliver the attachments as with version 1.2.0

I'm just the UI guy, but i think the called endpoint is this:

/boards/{boardId}/stacks/{stackId}/cards/{cardId}

{
    "title": "130 buggy",
    "description": "",
    "stackId": 505,
    "type": "plain",
    "lastModified": 1615300299,
    "lastEditor": "stefan",
    "createdAt": 1615300016,
    "labels": [],
    "assignedUsers": [],
    "attachments": [],             // → Here's the error
    "attachmentCount": 1,          // → LOL
    "owner": {
        "primaryKey": "stefan",
        "uid": "stefan",
        "displayname": "Stefan Niedermann",
        "type": 0
    },
    "order": 999,
    "archived": false,
    "duedate": null,
    "deletedAt": 0,
    "commentsUnread": 0,
    "id": 756,
    "ETag": "7273ce30eb6e6132d944bf27b6f6f181",
    "overdue": 0
}

Login as admin user into your Nextcloud and access
http://example.com/index.php/settings/integrity/failed
paste the results here.

If you have access to your command line run e.g.:
sudo -u www-data php occ app:list
from within your Nextcloud installation folder

If you have access to your command line run e.g.:
sudo -u www-data php occ config:list system
from within your Nextcloud installation folder

or

Insert your config.php content here
Make sure to remove all sensitive content such as passwords. (e.g. database password, passwordsalt, secret, smtp password, …)

Insert your Nextcloud log here

Insert your browser log here, this could for example include:

a) The javascript console log
b) The network log
c) ...

Start at the /boards/{boardId}/stacks/{stackId}/cards/{cardId} endpoint and trace how a card's attachments are assembled. Reproduce the case with Deck 1.3.0 and an attachment, then verify that the API returns the attachment data instead of an empty array and that it remains consistent with attachmentCount.