The content type of the file being downloaded.
This will initially be null, and will be populated as soon as the appropriate header is received from the server. Its value
will never change after this.
Note that change notifications for this property (#GObject::notify emissions) may be emitted in threads other than the one which created the DownloadStream. It is the client's responsibility to ensure that any notification signal handlers are either multi-thread safe or marshal the notification to the thread which owns the DownloadStream as appropriate.