I guess this post gives some kind of answer to your question.
I found it these days because I was also searching for an "extended text editor".
I want (like you) text file edit - and (like the other poster) CSS file edit.
Reading the linked post I decided not to use RadEditor in this case.
So I do two different things
a.) for Textfiles (small ones) I use a normal textbox
Spell is useable - and "formatting" on a textfile :)
b.) for CSS and other specialized filetypes I implemented a download / upload scenario with RadFileExplorer
This is OK for my customer - he uses Visual Studio Express Web for CSS - which gives "WYSIWYG" editing.
Ah, by the way - my HTML content editing is done with RadEditor.