WMTS format: image/png not available as output

I successfully configured a WMTS layer, however even when setting the format suffix to 'png' in the OL user interface, the map request (map preview) is always build with jpeg as output format.

Tip to the developers: please also make clear in the UI how to the format should be defined: sometimes you need to set 'image/png', but for TMS it is only 'png'