how to open .html file in rich text box

'HTML Format' data format not supported.<br />
Parameter name: dataFormat

richTextBox_LessonView.Document = new FlowDocument();
             TextRange documentTextRange = new TextRange(
                 richTextBox_LessonView.Document.ContentStart, richTextBox_LessonView.Document.ContentEnd);

             using (FileStream fs = File.Open("softskill.htm", FileMode.Open))

                 documentTextRange.Load(fs, System.Windows.DataFormats.Rtf);