A React port of the "Files - Drag-and-Drop" Elm Example.
When the user drags a file 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 files by clicking on the button instead. The labels in the example show the names of the last-uploaded files (i.e. files: ) and whether the user is currently dragging a file over the droppable area (i.e. hover: ).