Its copied on the bottom because its an example of a feature not finished on another (not really) WIP module.
it does what you are talking about. Copy image from another field, or from a screenshot, it doesnt matter, I understood you was talking about « clipboard ».
The specific reason is as you are limited on mimetypes when working with the clipboard write api, in the example above it render the blob in a canvas, on différent format, including webp.
Voila ?