Quick tip - each filety key will contain a subkey named "OpenWithProgids". The registry values listed under that subkey will represent all the prog IDs registered to open that extension. (i.e. under .pdf you should see FoxitReader.Document, AcroExch.Document.DC, and any other PDF reader/editor software you have installed.) This isn't necessarily a comprehensive list of all programs you could use - but it's a good start and should contain the programs that are safe to use.