uploadForm 上传返回json时报错，extjs不识别返回的json字符串，google以后，找到一个讨论帖：
http://www.sencha.com/forum/showthread.php?120201-Submitting-a-form-seems-to-be-mis-interpreting-the-response-type&
"try not setting the response to application/json. All "AJAX" file uploads must use iframes, thus a traditional form submit is required. The exception to this rule is use of the XHR2 object, which most browsers don't implement."
大意是iframe的提交方式与extjs默认的ajax方式不同，所以不要指定返回类型为 application/json，在django端把返回类型改成text/html就可以了