Occassionally there are problems recognising the MIME type of a file, so we should allow the user to specify the "type" property, which will override the discovered type in the EmbeddedResourceParser.

e.g. !http://traffic.alexa.com/graph?w=379&h=216&r=6m&y=r&u=google.com/&u=yahoo.com|type=image/jpg! will render the file as an image, even though there is no file extension in the URL.