Dragging and dropping an image from a web browser to Nautilus makes Nautilus encode its texture as a PNG instead of downloading it
With Nautilus 45.x under Wayland, drag-and-dropping image files from Epiphany (or Firefox) to Nautilus makes Nautilus create a whole new PNG for it (probably from the image texture), even if the image was already compressed (as JPEG or WebP or whatever).
Presumably, it should instead get it as a file list and download it, rather than converting it to heavier PNG.