I thought I had this all figured out! I've written an app with its own icons on which the user can drop any icon. I've included the BNDL as well as FREF resources for ****, fold, and disk. Yes, the BNDL bit is also set.
When installing this file on a new machine, the icon does follow but the app doesn't recognize itself as being "droppable" (i.e. won't highlight). Rebuilding the desktop fixes the problem - but why should I have to???
What am I missing here?