There is a nice option in MacOS X that you can right-click on a particular file, select "open with", then select the app that you want to use to open the file type and check the mark "always open with".
So far so good, this works very well. However, this association resets itself after time (or after a system restart for example). How can I achieve it that a particular file type stays associated with my selected "always open with" app?