EricB - you open my computer, head to tools, folder options. click on the file type tab. you will to find your extension. then delete it. then click new. put that file extension back on. now choose "open with" . pick your porgram to open it with
That's only if you want to perminantly associate a file type with an application, and is not a replacement for the right-click-menu option of "Open-With" since that is not what that is there for.
The "Open With" command from the RC-menu is meant to give you the option of opening the file with a program other than what is set by default. This is useful if you have multiple graphics programs, sound programs, video programs, etc, since you may not always want to open your images/movies/sounds/etc with the same app all the time.
The folder-options method changes the default of that file so that the icon, functionality, and application started by that file, is changed perminantly. This is often messy and not usually recommended. Proceed with planning...