request.parsePostForm() は Content-Type が無ければ application/octet-stream と見なす様だ

func parsePostForm(r *Request) (vs url.Values, err error) {
:

ct := r.Header.Get("Content-Type")
// RFC 7231, section 3.1.1.5 - empty type
// MAY be treated as application/octet-stream
if ct == "" {
ct = "application/octet-stream"
}
ct, _, err = mime.ParseMediaType(ct)
: