You can take use of
Filters to show only DOC and TXT files
OpenFileDialog dialog1 = new OpenFileDialog();
dialog1.Filter = "txt files (*.txt)|*.txt| DOC files (*.doc)|*.doc";
you can load files in Richtextbox, by using
LoadFile method
richTextBox1.LoadFile(FilePath, RichTextBoxStreamType.RichText);
richTextBox1.LoadFile(FilePath, RichTextBoxStreamType.PlainText);