A ContentSniffer tries to detect the actual content type of the files that are being downloaded by looking at
some of the data before the [class@Message] emits its [signal@Message:SoupContentSniffer:got-headerss] signal.
ContentSniffer implements [iface@SessionFeature], so you can add content sniffing to a session with [
[email protected]_feature] or [[email protected]_feature_by_type].