The third check for contentType of image has a serious bug, the current code is as follows:

if (!contentType.startsWith("image")){
    response.setStatus(404);
    log.info("httpProxyServlet returning response 404 after receiving element with contentType ="+contentType);
}

However, it should be:

if (!contentType.startsWith("image")){
    response.setStatus(404);
    log.info("httpProxyServlet returning response 404 after receiving element with contentType ="+contentType);
    return;
}

Without the return any content is still retrieved and displayed.