原因：后台返回的json格式没有严格按照github中的格式返回
参考：https://groups.google.com/forum/#!topic/jquery-fileupload/0q8PN2v0I28
https://github.com/blueimp/jQuery-File-Upload/wiki/Setup#using-jquery-file-upload-ui-version-with-a-custom-server-side-upload-handler
https://blueimp.github.io/jQuery-File-Upload/
http://nuxseme.com/js/Jquery-File-Upload/
http://jsbin.com/tiwuketuxo/edit?html,output（不断修改中）