private static MediaType createConstant(String type, String subtype) {
        MediaType mediaType =
                addKnownType(new MediaType(type, subtype, ImmutableListMultimap.of()));
        mediaType.parsedCharset = null;
        return mediaType;
    }
Matcher for type, subtype and attributes.
core/src/main/java/com/linecorp/armeria/common/MediaType.java