Some modern website these days use WebP format in an effort to make things faster. Although great for browser display it might be a pain for you to edit or even to upload to another website. Older OSes also can't open the file natively and can't display the preview.

To save yourself the hassle of converting the images later, just use the Chrome extension Save Image As You Want. Just right click and you can choose the desired file format - JPEG or PNG. You don't feel any noticeable lag in converting and saving directly when using this extension so it's such a time saver.