The following question came in from a customer:

If our program isn’t installed and users double-click our document, they get sent to a Web site that presents a list of programs, but we want to send the user directly to our download site. How do we claim a file extension for our application?

Um, you don’t.

You cannot pre-emptively reserve a file extension. If your program uses the extension .ABC and somebody off in another country working out of a garage also uses the extension .ABC, then that’s fine. Of course, things get exciting when a user installs both your program and the garage program, but that’s a conflict for the user to resolve. It is not your position to declare unilaterally that you are more important than that person in a garage.

What you can do is list your program among the ones that support the .ABC extension, so that when the user double-clicks an .ABC document and no program is installed to handle it, the user is sent to a Web page that includes your program as one which can open that file. But this doesn’t grant you exclusivity. You’ll just show up on the list with everybody else who wants to claim that file extension.