Drag and drop
From desktop to application
This is a feature that is supported by HTML5. See the relevant MDN docs on File drag and drop.
From application to desktop
Dragging something out of a Miletus application is supported but the file must already exist on the local file system.
If the file does not exist it's up to the developer to create it on the fly (and clean the temporary file afterwards if needed).
If the file exists then the file dragging can be started by calling
files.startDrag with the path to the existing file: