This is a continuation of efforts from
LPS-87348, focusing on the UX aspects requested by the Lexicon team.
When a user is attempting to drag and drop an image file into a WYSIWYG editor, from the OS file system, the UI should hint/highlight what valid drag targets are available.
See "Valid Target" at: https://lexicondesign.io/docs/patterns/drag_and_drop.html
Also see Emiliano's comments for some background.