[Vince]

If you try to open a file with an extension that Windows does not know, it will ask you which program to open it with. There will be a checkbox like 'Always open these files with this program'. If that is checked, the popup will no longer appear if you try to open the file again. I don't really see what your problem is.

You can enable all file extensions by going into Windows Explorer. Go to Extra -> Folder Options -> Display and untick the box that says 'Hide file extensions for known file types'. Note that I use a Dutch version of Windows, so translations may not be accurate.