在可绑定的RichTexBox mvvm wpf中加载rtf

I'm new to mvvm and I would like to load an rtf file in a RichTextBox using mvvm, but the text doesn't seem to display in my richtextbox. Looks like RichTextBox is pretty complex to deal with when trying to place the commands in the ViewModel. I'm not sure where I go wrong.I'm new to mvvm and I would like to load an rtf