Autostack file type filter

Today, if you run Autostack on a folder with Raw + Jpeg pairs, it will correctly ignore the Jpeg files as long as they have the same filename stem. But some cameras give the files different stems. So if the user picks a folder that contains multiple file types, we should give them the option to only stack files of a selected type. This picker doesn’t need to show up if the folder contains only one type.

Originally suggested/reported by Paul W.