If I am not wrong the content type has to be defined from wordpress, not at your xml. The httpcontext you are trying to access has no sense. You only provide an xml file, not processing a web request.
If wordpress does not recognize your feed then there may be other reasons, may be format error or their side wordpress not properly configured.
You may validate your feed through a free validator . One is here
http://validator.w3.org/feed/[
^]. Google helps you to find more and find out what error with your feed. If your feed has no error then you need to check wordpress configuration.