And are you sure they were all real PNGs? You know, displayed suffix means nearly nothing nowadays, you can serve JPEG image as ".png" file with "image/png" mime header and browser would not even raise its eyebrow and process the JPEG properly.
I occasionally archive some imagery from web and while viewing them in Irfan I am frequently confronted with "This image is JPG with .png extension, would you like to rename it?" prompt, without what I'd probably had no idea how frequently this discrepancy occurs.
I occasionally archive some imagery from web and while viewing them in Irfan I am frequently confronted with "This image is JPG with .png extension, would you like to rename it?" prompt, without what I'd probably had no idea how frequently this discrepancy occurs.