Fix for dragging/dropping multiple files in QS

Fixes #1866

So `public.file-url` data on the pasteboard can only hold the info for ONE file copied/dragged. We still need to use `NSFilenamesPboardType`. Even Finder does this: drag 2 files from finder; the pasteboard data for `public.file-url` refers to one of the files only, and the `NSFilenamesPboardType` refers to both