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/1.1

Host: localhost

Connection: keep-alive

Content-Length: 9

Origin: localhost

User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36

Content-Type: application/json

Accept: */*

X-Requested-With: XMLHttpRequest

Referer: http://172.17.35.112:8099/

Accept-Encoding: gzip, deflate

Accept-Language: zh-CN,zh;q=0.8

Cookie: selectFluence=4; VFS_USERNAME=admin; VFS_PASSWORD=123456; VFS_APPURL=; VFS_ISSAVE=true; VFS_ISDMZ=true; webserver_is_save=0; _alert=1495876699555

aa=1&bb=2