Use content-type header for the svg hint in NeonImage

[Leptopoda]

NeonImage currently guesses the image type by trying to decode it utf8 which might not even be the right encoding.
Use the content-type header as it correctly conveys this information.

This is currently not possible as the RequestManager swallows the headers for our binary requests. This probably depends on the request manager being inlined into the httpClient and other work related to #2155