I'm working with a WCF restful/http method that returns a stream of image data. I want to make sure that the content type is marked as "image/png". The method is defined as:

[ServiceContract]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
public class TileImageService
{
    [WebGet(UriTemplate = "{id}")]
    public Stream GetTileImage(string id)
    {
        Bitmap bmp = new Bitmap(173, 173);
        Graphics g = Graphics.FromImage(bmp);

        g.Clear(Color.Blue);
        g.DrawString(DateTime.Now.ToLongTimeString(), new Font("Chiller", 20), Brushes.White, new PointF(10, 10));
        g.Flush();

        MemoryStream ms = new MemoryStream();
        bmp.Save(ms, ImageFormat.Png);

        ms.Seek(0, SeekOrigin.Begin);

        return ms;
    }
}

In Firefox it looks like the content type is marked as application/octet stream. Is there a way to change the content type?