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/paths/application/conversation/messages/create_attachment.yml

@@ -31,7 +31,8 @@ post:
       type: array
       description: The files to be uploaded.
       items:
-        type: file
+        type: string
+        format: binary
 
 
   responses:

swagger/paths/application/conversation/messages/index.yml

@@ -6,12 +6,13 @@ description: List all messages of a conversation
 responses:
   200:
     description: Success
-    type: array
-    description: Array of messages
     schema:
-      allOf:
-        - $ref: '#/definitions/generic_id'
-        - $ref: '#/definitions/message'
+      type: array
+      description: Array of messages
+      items:
+        allOf:
+          - $ref: '#/definitions/generic_id'
+          - $ref: '#/definitions/message'
   404:
     description: Conversation not found
   401: