Okay, I don't think it can be done with PocketPlus. Folders don't have file extensions so I couldn't change file associatons for folders.
Have you looked at PPX3K? It's another Today Screen Enhancer but it's more hands-on that is to say you get more/have to set more options to set it up. Look on
www.ppx3k.com, try the trial and let us know what you think. Personally it's to much work to setup to me but some folks like it.