How do you change the default opening application?

[marc]

Changing the default app for an exisiting mime-type is simple, but
how do you change the default app for "undefined" mime-types?

When I open a file in a file manager, I want it to open in
the appropriate application -- and most of the time it does. When the
file's mime-type does not have an associated app, I want to open the
file with my application of choice. How do I change this application?

The problem I have at the moment is that, somehow, the .chm reader
application that I have installed has become the "fall back"
application, which is not a good thing.

-- 
Best,
Marc

"Change requires small steps."