Anytime I upload an image to the media library it shows up as a png. And when I want to use it in a post WordPress tells me I can’t use a png.

How does this happen? The site is in development and running on localhost.

I have changes the .htaccess to allow bigger files to be uploaded. But after deleting these rules nothing changes. Nothing in functions.php about the media library.

How does this happen? Where should I look to troubleshoot this?