自己写的一个webpack中间件转发请求，有如下错误

Question

核心代码如下

middleware(req, res, next) {
    server = req.query.server;
    server += '/index.html';
    ...
    // 准备好的reqOpt
    let req = https.request(reqOpt, proxyRes => {
        let data = new Buffer(0);
        proxyRes.on('data', chunk => {
            data = Buffer.concat([data, chunk]);
        });
        proxyRes.on('end', () => {
            let fileContent = fs.readFileSync(path.join(__dirname, 'page/index_dev.html'));
            res.setHeader('Content-Type', 'application/json');
            res.end(fileContent);
        });
    })
    req.end();
}

接口控制台报错如下：
EventSource's response has a MIME type ("application/json") that is not "text/event-stream". Aborting the connection.