A binary or rather a base64encoded responses requires
"Binary Media Types" to be configured in API Gateway
a request with an "Accept" header set to one of the "Binary Media
Types"
see https://api.slack.com/actions
see https://github.com/slackapi/bolt/blob/dae21827c1c11720e5d6c8f23abcddb2d983b1f2/src/types/actions/message-action.ts