@Override
    public HttpRequest toHttpRequest(RequestHeaders requestHeaders) {
        final RequestHeaders requestHeadersWithBoundary = injectBoundary(boundary, requestHeaders);
        return HttpRequest.of(requestHeadersWithBoundary, this);
    }
Returns a random boundary used for encoding multipart messages.
core/src/main/java/com/linecorp/armeria/common/multipart/DefaultMultipart.java