微信小程序下载文件时，totalBytesExpectedToWrite为null怎么办？

解决办法：

URL path = new URL(url);
HttpURLConnection connection = (HttpURLConnection) path.openConnection();
// 获取文件总长度
long contentLength = connection.getContentLengthLong(); 
// 设置Content-Length响应头部
response.setHeader("Content-Length", String.valueOf(contentLength));