Hi Adrian,
I'm Christine, a Windows user like you. I'll do my best to help you. I can see here that you're getting an icon of .png when downloading or creating a photo.
As per checking.png is the default file type of Photos on Windows 11.
May I ask you to send screenshot of the issue that you're getting?
Send Screenshot
https://answers.microsoft.com/en-us/feedback/fo...
Will wait for your response. I'm just here.
Thank you,
Christine L
Independent Advisor