public MultiPartStreamer addData(String name, String type, String filename, Supplier<InputStream> data) {
        streams.add(() -> separator(name, filename, type));
        streams.add(data);

        return this;
    }
Adds the given data as a named part in this, using the given content type.
hosted-api/src/main/java/ai/vespa/hosted/api/MultiPartStreamer.java