A careful reading between the lines of the [API Gateway documentation][1]
implies that if `binaryMediaTypes` is not set on the API Gateway, all
responses are considered to be text. So we should be able to get away
with an explicit list of media types that require binary responses,
and we can ask the developer to ensure that it matches the
`binaryMediaTypes` setting on his or her API.

[1]: https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-payload-encodings-workflow.html