Proper use of undocumented attachment_ids field in side conversations API.

I am part of a team developing an application installed to the ticket_sidebar location. As part of this we want to be able to programatically create side conversations for our users.

We've had success doing by calling the side conversations API via ZAFClient:

https://developer.zendesk.com/api-reference/ticketing/side_conversation/side_conversation/#create-side-conversation

Our code is essentially this, which works: 

const { 'ticket.id': id } = (await client.get(
  'ticket.id',
));

const message = {
  "message": {
    "subject": "My printer is on fire!",
    "body": "The smoke is very colorful.",
    "to": [
      { "email": "someone@example.com" } 
    ],
  }
};

await client.request({
  contentType: 'application/json',
  url: `/api/v2/tickets/${id}/side_conversations`,
  type: 'POST',
  data: JSON.stringify(message),
  dataType: 'json',
})

We would also like the ability to add attachments. This is where we run into issues.

The example code from the documentation would suggest this is possible:

{
  "message": {
    "subject": "My printer is on fire!",
    "body": "The smoke is very colorful.",
    "to": [
      { "email": "someone@example.com" }
    ],
    "attachment_ids": ["0123456789"]
  },

However the attachment_ids field is not documented on the associated Message object, and in testing always results in an error that the attachment was not found.

https://developer.zendesk.com/api-reference/ticketing/side_conversation/side_conversation_event/#messages

Our expectation was that we could get a list of attachments by first making a request to /api/v2/tickets/${id}/comments.json , then use the attachment IDs from that as part of the message payload. (We've also confirmed work with the api/v2/attachments/ endpoint). Inspecting the GraphQL network requests that are made when creating a side conversation via the UI also results in the same attachment IDs being used - so it is unclear why these attachments aren't being found.

TL;DR - is the attachment_ids field actually supported in the side conversations API, and if so what do we need to do to get this to work?