When you create an image in the now old Affinity Designer you specify how you want it saved. I use SVG in Designer and PNG in Photo. I played around a bit with V3 under its new Canva ownership and it seems no different with respect to file management. The new V3 has everything organized into one package (and there are some neat new bells and whistles). V3 created png and SVG images added to Zazzle just fine. I never save files in the Affinity format so .AF is irrelevant for me. Just waiting for the iPad version since I now rarely use a laptop.
