Attachments: extendedData is an Array in some Cases

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Describe the bug
We have a user experiencing a bug, where the server returns the extendedData of an attachment as an array instead of an object:

{
  "title": "Melanie Webseite",
  "description": "http://someDomain.ch",
  "stackId": 6,
  "type": "plain",
  "lastModified": 1583477447,
  "lastEditor": "timo",
  "createdAt": 1583441985,
  "labels": [],
  "assignedUsers": [],
  "attachments": [
    {
      "cardId": 162,
      "type": "deck_file",
      "data": "someDomain-Einladung-feriegeburtstagswuche.jpeg",
      "lastModified": 1583442122,
      "createdAt": 1583442122,
      "createdBy": "timo",
      "deletedAt": 0,
      "extendedData": [/*THIS_ONE*/],
      "id": 13
    }
  ],
  "attachmentCount": 1,
  "owner": {
    "primaryKey": "timo",
    "uid": "timo",
    "displayname": "Timo SomeSurname",
    "type": 0
  },
  "order": 999,
  "archived": false,
  "duedate": null,
  "deletedAt": 0,
  "commentsUnread": 0,
  "id": 162,
  "overdue": 0
}

To Reproduce
Steps to reproduce the behavior:

1. Sync via Android App with unknown constellation (sorry for that!)

this happened after an NC upgrade from 18 to 19

Expected behavior
extendedData always to be an object

Client details:
(quite irrelevant -> REST!)

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) ...

No source file or test is identified. Start by reproducing the REST response after an Android sync and trace the attachment serialization path responsible for extendedData. Done means the API consistently returns extendedData as an object, with a regression test covering the affected response.