jquery ajax contentType是啥意思？

Question

POST请求contentType设置为application/json，但请求却把data的json转成了字符串？请大神指教是什么原因？代码如下请求抓包POST http://localhost:8888/demo_test.txt HTTP/1.1Host: localhostConnection: keep-aliveContent-Length: 9Origin: localhostUser-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36Content-Type: application/jsonAccept: */*X-Requested-With: XMLHttpRequestReferer: http://172.17.35.112:8099/Accept-Encoding: gzip, deflateAccept-Language: zh-CN,zh;q=0.8Cookie: selectFluence=4; VFS_USERNAME=admin; VFS_PASSWORD=123456; VFS_APPURL=; VFS_ISSAVE=true; VFS_ISDMZ=true; webserver_is_save=0; _alert=1495876699555aa=1&bb=2