Function extract_mime_type_as_dataurl_mime 

pub fn extract_mime_type_as_dataurl_mime(headers: &HeaderMap) -> Option<Mime>

https://fetch.spec.whatwg.org/#concept-header-extract-mime-type This function uses data_url::Mime to parse the MIME Type because mime::Mime does not provide a parser following the Fetch spec see https://github.com/hyperium/mime/issues/106