OptionalcontentThe Content-Type of the callback requests initiatiated, It supports application/x-www-form-urlencoded and application/json, and the former is the default value.
OptionalcustomOptionalheadersextra headers, detail see RFC 2616
OptionalhostThe host header value for initiating callback requests.
After a file is uploaded successfully, the OSS sends a callback request to this URL.
The value of the request body when a callback is initiated, for example, key=$(key)&etag=$(etag)&my_var=$(x:my_var).