Extracting cover art, change *.png (default) to *.jpg (default)

I would like to add to my folders a cover art file extracted from embedded iTunes MP3 files.
When I click on the picture of album art the following menu proposes automatically (nine times out of 10) *.png as default file format.
How can I change this default format to a new default format *.jpg?

Mp3tag exports the file in the same format it has been previously added to the file. You can open it with your favourite image editing tool afterwards and save it in the desired format.