When I uplaod a favicon, it need to be a png. Why? Every favicon is .ico!
Historically, Microsoft decided to use the ico-format for favicons in Internet Explorer 5 in 1999. There are not as many ico-editors as there are png-editors which is why we decided to go with png instead: it's easier for our users and allows for better quality icons. By the way, not "every favicon is .ico". There are plenty websites out there that use gif or png.
You can do anything
I think I was way to harsh on that day, sorry for that!