Major Description
getContentType() uses a class member variable to track if a content type has been set. The problem is the MimeResponseImpl doesn't monitor the set/add property calls which according to the spec can also set HTTP headers including the Content-Type header.
So if I a client sets the Content-Type via a setProperty call and then calls getPortletOutputStream or getWriter the MimeResponseImpl will assume no content type has been set and set the first value from getResponseContentTypes() as the content type, overwriting the previously requested value. If MimeResponseImpl checked the PortletMimeResponseContext.getContentType it could allow for portals that handle all HTTP headers.