From what I’ve read and experienced, file extensions are hidden by default in native file dialogs and users can change a setting to then show them. In recent Windows and Mac the setting is written something like “Show extensions” with a checkbox, though at least in Windows 7 in an advanced area it is referred as the opposite as “Hide extensions” and that is checked.
- For Windows, extensions are hidden in Explorer by default (see Article telling users how to show them.)
- For macOS, extensions are hidden in Finder by default (see Article telling users how to show them.)
I think it would be best for Slicer to expect that users are familiar using the defaults of the major OS platforms. I would be in favor of Slicer’s save dialog not requiring the extension to be specified by default and letting the autoselection of File Format to be used as the extension. The Slicer save dialog could have a user setting to then show extensions in the file name and that would reimplement the option that is present for native file dialogs on the major OS platforms.