I'm not quite sure. The default extension is .jpg (hard coded). The use of IrfanView ticked in your options converts them automatically into jpg. Even if you leave this field unticked the extension still has to be .jpg. Maybe it will be shown if it's in real a gif. But animated gifs won't be viewed - except of the first frame.
In order to achieve what is possible, you have to try the impossible over and over again. Hermann Hesse