Index is just the file name on the remote site (Google's cache), it just misses the extension to view properly in Windows Explorer, try to save it as image.png. However Google Images shows only thumbnails to prevent copyright issues, you should visit the original site or use a different search engine like DuckDuckGo for image search.

The screenshot you posted could indicate that this image in a SVG image (i.e. a text file with SVG code) and not a normal image with a .jpg or .png or other valid image file extension.

web images from Google save as a .webp which is far from useful. Can I save as png or jpeg?

See:

• WebP image converter https://addons.mozilla.org/firefox/addon/webp-image-converter/