How do I know if the user should have access to the insides of directories with certain extensions?

FROM : Alan Smith
DATE : Tue Aug 15 20:48:28 2006

For example:

The user should be able to open anything with the ".local" extension
but not anything with the ".rtdf" extension. By "open" I mean, the
user should be able to view its contents without selecting something
like "Show Package Contents." Just double-click and it opens and the
user is then looking at the contents. As opposed to double-clicking
and an app tries to open it.

I hope you aren't really confused about the question, I did my best.
Thanks, Alan

// Quotes from yours truly -------------------------
"You don't forget, you just don't remember."
"Maturity resides in the mind."
"Silence is the Universe's greatest gift."
"Don't waste your life doing things others have already done."