Define a key for response content type

Define a key in `api.toml` associated with a route that specifies the content type or types including at a minimum Octet-stream, HTML, and JSON. For consistency with the HTTP specification, probably best to call this content-type, though maybe better to match the capitalization and punctuation of other keys in api.toml.

Start by locating the route definitions in api.toml and reviewing the existing key naming and capitalization conventions. Define the response content-type key so it can represent Octet-stream, HTML, and JSON, with the route configuration recognizing the new value; done means the key and its supported types are unambiguous and consistent with the HTTP specification.

Written by the indexing model from the issue text.