Using the specific file extension will return only those files wanted. As others say, use a path to the desired directory instead of trying to cd all over the place. You don't provide enough info to know whether your location is relative to working directory or not so the minimum you can get away with isn't possible to say for the search directory. dir() returns a fully-qualified directory regardless of whether the search is absolute or relative.
You don't have to worry about the . or .. entries with this way with the one caveat you've not named a directory with a .plx extension. If you have, you deserve the grief... :)