The UI for the extension filter takes up the full width of the window:
Compare that to Chrome's save dialog, for example:
MCVE: https://github.com/deNowww/MCVE-FileChooserExtensionFilterSaveDialog
as far as I know, it is a known issue since 2018 (see here). you may track this issue to see if they are ever gonna fix it or when they decide to fix it.