有些版本的flash插件，不支持POST请求的回包body为空的情况。

对此的规避办法，也就只能输出点什么东西好了（AWS的做法只是一个参考，忽略设置HTTP status 201）。

Some versions of the Adobe Flash Player do not properly handle HTTP responses that have an empty body. To configure POST to return a response that does not have an empty body, set success_action_status to 201. Amazon S3 will then return an XML document with a 201 status code.

via: http://docs.aws.amazon.com/AmazonS3/latest/dev/HTTPPOSTFlash.html#HTTPPOSTAdobeFlashConsiderations