@Override
    public HttpRequest toHttpRequest(String path) {
        requireNonNull(path, "path");
        final MediaType contentType = DEFAULT_MULTIPART_TYPE.withParameter(BOUNDARY_PARAMETER, boundary);
        final RequestHeaders requestHeaders = RequestHeaders.builder(HttpMethod.POST, path)
                                                            .contentType(contentType)
                                                            .build();
        return HttpRequest.of(requestHeaders, this);
    }
Returns a random boundary used for encoding multipart messages.
core/src/main/java/com/linecorp/armeria/common/multipart/DefaultMultipart.java