A React port of the "Files - Image-Previews" Elm Example.
When the user drags an image from their computer on top of the dashed area, the area will change colors, indicating that the file will be uploaded once the user drops it there. The user can also select a list of images by clicking on the Upload Images button instead. A small thumbnail preview of the uploaded image(s) will be visible in the dashed area after selecting from the button's menu, or after dragging images onto the area.