插件：
https://github.com/blueimp/jQuery-File-Upload/wiki/Chunked-file-uploads
https://blog.csdn.net/sinat_29179927/article/details/89059919
formdata解释：
https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest#Submitting_forms_and_uploading_files
https://www.cnblogs.com/xiaohuochai/p/6539330.html formdata序列化
https://www.jianshu.com/p/e984c3619019 formdata方法