Fix: Make swagger doc compliant to OpenAPI (#3394)

This fixes issues in the swagger.json file. The motivation to do so is to be able to generate API clients using https://openapi-generator.tech Doing so will require further changes to the api spec, but this seems like a good first step since it is now "valid" according to editor.swagger.io and openapi-generator validate.

Fixes #2806

swagger/definitions/resource/public/message.yml

@@ -1,5 +1,5 @@
 type: object
-properties: 
+properties:
   id:
     type: string
     description: Id of the message
@@ -23,6 +23,8 @@ properties:
     description: Conversation Id of the message
   attachments:
     type: array
+    items:
+      type: object
     description: Attachments if any
   sender:
     type: object