http-server-js: handle raw bytes responses with unknown content-types and Http.File response bodies (#10799)

This pull request corrects the handling of `bytes` responses when they
should be treated as _raw_ bytes while preserving JSON-encoding of bytes
payloads. It also implements support for `Http.File` responses with
arbitrary content-types and implements setting the `filename` using the
content-disposition header (we always use the "attachment" disposition).