I removed .png from a test file and it worked for me. It may be the mime type it’s being sent as on your machine. @kelciour, do you happen to know if the trick you mentioned on Anki is unable to display any SVG images on my computer - #12 by kelciour also allows changing the mime type for files without any extension?
But that said, you’ll probably find it easier to give the filenames an extension. You can use a bulk renaming program if you have a lot of them.