Add an option that if checked always overrides the import extension when exporting

Environment/Versions

• GIMP version: 2.10.28
• Package: Ubuntu 20.04.3 LTS
• Operating System: Linux

Description of the bug

The "Default export file type" as set in Preferences is not what appears in the Export Image file dialogue of "Export as" leading to the tedious job of manually changing the file extension every time I do an export.

Reproduction

Is the bug reproducible? Always

Reproduction steps:

1. Go to Edit > Preferences > Image Import & Export > Export File Type
2. Change the "Default export file type" to "JPEG Image" and click OK
3. Open a PNG file
4. Go to File > Export as or use the Ctrl+Shift+E shortcut
5. Look at the chosen export file extension
6. Click Export button and look at the file type dialogue

…

Expected result:

1. the file extension to be jpg or jpeg
2. the Export Image dialog to be for JPEG

Actual result:

1. the file extension is png
2. the Export Image dialog is for PNG