Looking good! If you’re open to a small suggestion, and if fiatjaf doesn’t mind introducing some early-stage backwards-incompatible changes to khatru's Blossom’s API, I think you could simplify things by changing the return type here https://github.com/fiatjaf/khatru/blob/76ecf4f7914a93b7ec4f0cc0c823304a7402adab/blossom/server.go#L18 to `io.ReadSeeker`. This would allow you to eliminate the `io.Copy` fallback code and the `Content-Type` handling, as `http.ServeContent` also takes care of  that for you. It even handles ETags and cache-related headers, so I’d expect your patch to significantly improve things for all clients—not just iOS. Well done!