Content type for posts and puts.
If this property contains a non-empty string, a Content-Type HTTP request header is added to the request. The purpose of the header is to show the contents of the data during a Post or Put to the server.
The most common example is posting of HTML form input data. In that case, this property must be set to "application/x-www-form-urlencoded".
This property is not available at design time.