External Editor FIle association

I've got something that I cannot figure out the answer to.
I want to use Stylus Studio to edit my xml/xsl files. I added it into the external editor portion, and associated it with those file types. However, when I got to open one of those type files the internal editor fires up. I've gone as far as to take out the internal editor but it still comes up.

Any ideas on where to look?
thanks in advance

Please post a screenshot of your "editor preferences".