Thanks for the deleting suggestion.
I had to delete the association, then go through advanced mode to set it to "open" blah\blah\pn.exe with %1 (file path) in the command line, which I also had to surround by quote marks by myself.
I had to delete the association, then go through advanced mode to set it to "open" blah\blah\pn.exe with %1 (file path) in the command line, which I also had to surround by quote marks by myself.