Re: Use filename extension instead of content type to choose default application ?
Install 'File Types Editor' a.k.a. assogiate.
Add a new file type. You can give it a number of attributes. The most important is the filename, which should be *.<your extension here>.
File name patterns take precedence over file contents or XML elements. So even if the MIME susbsystem detects the file as XML, it should identify your file correctly. Of course, a more elegant way would be to specify XML elements for your file type but that can be tricky.
One your new file type is created, you can associate an application with it from within Thunar or Xfce4's MIME type editor.
husband@wife$ make sandwich
husband@wife$ sudo make sandwich