Thanks for the quick replies. But I'm afraid that there may be more to this than the screenshot from Bhikkhu Pesala shows. Perhaps it's operating system dependent? (I'm using Windows 10, including the big update they pushed me a few days ago, so build 10586.)
The screenshot below shows what I see when I click File -> Export, then select PNG from the row of icons and hit the Export button. I don't see the PNG file indication at all, just the Affinity Files.
And in the process of making you this screenshot, I discovered another issue that is a bit more than cosmetic. I saved the screenshot with the Windows snipping tool as a JPG, then opened it in Designer to experiment a little more. I then exported it as a PNG. When I did, AD said the file already existed, even though what actually existed was Capture.JPG, not Capture.PNG. I let it replace the file to see if it actually would, and it did. When I opened the file in InfanView to look at it, InfanView identified it as a PNG file with a JPG extension and offered to rename it. This could be problematic if I wanted to save multiple formats of a file in the same folder.