Kustomer api customer/search for messages with attachments

This is a kustomer api specific question.

Having Kustomer api endpoint https://api.kustomerapp.com/v1/customers/search, docs are here. I want to receive messages with attachments (attachment name, contentType, contentLength).

The body for this request (simplified version):

{
  'and': [
    {
        'message_channel': {
            'equals': ['email']
        }
    }
  ],
  'queryContext': 'message'
}

The problem here - the standard response does not include all the info about attachments that I need. The question is - Is there any documentation on request body params aggs and fields(I think they may help)? Is it possible somehow to change the request body, so that message attachments are included in response as well?

I am trying to minimise the number of calls to kustomer api - get all the info in one go.

Thank you for your time.