http://www.ruanyifeng.com/blog/2012/09/xmlhttprequest_level_2.html
http://www.cnblogs.com/yuanlong1012/p/5127497.html
https://front-js.cc/test/fileupload/
http://www.ruanyifeng.com/blog/2012/08/file_upload.html
原文：http://www.cnblogs.com/zhshlimi/p/6070637.html