Qt based file picker (only used in linux) doesn't support adding extensions to files based on the extension.

[dalthviz]

...This is a Spyder wide problem that will happen with every save dialogue on linux not just with the data explorer. The qt file picker(only used in linux) doesn't support adding extensions to files based on the extension.

To fix it we could:

1. Contribute the feature upstream to qt https://bugreports.qt.io/plugins/servlet/mobile#issue/QTBUG-27186

2. Write a wrapper around the save file dialogue function to add the extension from filterSelected and check for over writes. I couldn't get this to check the name with the file dialogue still open. It would be annoying to click no to the overwrite and then be left without a saved file of a dilague to change it.

3. Figure out how to get native(at least gnome and kde) file selection dialogues on Linux through anaconda and not use the silly qtfiledialogue that rarely actually ever gets used.(I would absolutely love this)

4. Do something else that I didn't think of :-)

This is why I haven't fixed this yet. I hope this helps you find a few directions to look.

Follow-up issue after merging PR #17790

Originally posted by @bcolsen in https://github.com/spyder-ide/spyder/issues/7196#issuecomment-663282342