it could be a chrome-related issue, I tried in SU, same error, while on firefox, it’s all clear.
And then I remembered having issues with JPG a few years ago, solved by renaming them jpg
So I tried. PNG in capital letters is refused, but simply renaming it png works.
You’ll find similar situation in various softwares, a quick search :
https://www.reddit.com/r/HTML/comments/jf5jeg/question_about_png_vs_png/
according to reddit, it’s an issue of case sensitivity.