@Nullable
    private SerializationFormat determineSerializationFormat(HttpRequest req) {
        final HttpHeaders headers = req.headers();
        final MediaType contentType = headers.contentType();

        final SerializationFormat serializationFormat;
        if (contentType != null) {
            serializationFormat = findSerializationFormat(contentType);
            if (serializationFormat == null) {
                // Browser clients often send a non-Thrift content type.
                // Choose the default serialization format for some vague media types.
                if (!("text".equals(contentType.type()) &&
                      "plain".equals(contentType.subtype())) &&
                    !("application".equals(contentType.type()) &&
                      "octet-stream".equals(contentType.subtype()))) {
                    return null;
                }
            } else {
                return serializationFormat;
            }
        }

        return defaultSerializationFormat();
    }
Returns the default {@link SerializationFormat} of this service.
thrift/thrift0.13/src/main/java/com/linecorp/armeria/server/thrift/THttpService.java