private InputStream separator(String name, String filename, String contentType) {
        return asStream(disposition(name) + (filename == null ? "" : "; filename=\"" + filename + "\"") + type(contentType));
    }
Returns the separator to put between one part and the next, when this is a string.
hosted-api/src/main/java/ai/vespa/hosted/api/MultiPartStreamer.java