Go言語のHTTPは勝手にContent-Typeを付与することがありそう
func DetectContentType(data []byte) string
DetectContentType implements the algorithm described at https://mimesniff.spec.whatwg.org/ to determine the Content-Type of the given data. It considers at most the first 512 bytes of data. DetectContentType always returns a valid MIME type: if it cannot determine a more specific one, it returns "application/octet-stream". 無効化したいがどうすれば。