Retrieve file extension from mime type of url

Is there a way to get a filetype extension from a MIME type (which includes charset as well) without writing long lists of file extensions and mime types?

Also a minor issue; how do I breakstrings separated by a character? For example "text/html; charset=ISO-9859-1" I want to return both "text/html" and "charset=ISO-9859-1" as separate strings.

 private void saveFileDialog1_FileOk(object sender, CancelEventArgs e)
        {
            var request = System.Net.HttpWebRequest.Create(((WebTab)ActiveMdiChild).geturl()) as System.Net.HttpWebRequest;
            string contentType = "";
            if (request != null)
            {
                var response = request.GetResponse() as System.Net.HttpWebResponse;
                if (response != null)
                    contentType = response.ContentType;
            }
            //I can only retrieve html pages at the moment
            if (contentType.Contains("text/html"))
            {
                System.IO.File.WriteAllText(saveFileDialog1.FileName, ((WebTab)ActiveMdiChild).getsource(),Encoding.Unicode);
            }
        }

Ivan