I have a command-line Python script that works well to convert one sort of file into another given a few parameters and would now like to deploy this to some of my colleague
Basically you just need to figure out what widgets will hold the data you want the best. I suspect you could use a couple combo boxes to hold different sets of extensions. Or you could just use the path name strings to figure that out. Hit a button and run the conversion process, probably in another thread so the GUI remains responsive.
I'm biased for wxPython. However, you're better off taking a look at their demos and documentation and seeing which GUI toolkit fits your brain the easiest.