With the release of v0.6.0, Boomla only lets you drop files where they are accepted.
When dropping a file, some of the drop zones will turn red. It means the dragged file can’t be dropped there. If all drop directions (before, in, after) relative to a file are invalid, it won’t react as droppable at all. If at least one direction is valid, it will show up as blue, all the other drop directions will react as red. So, if you see a red drop-zone, you can tell that another drop direction is valid on the same target.
See the .Accept file docs.
In the IDE, you will see hints only. If you shouldn’t drop a file there, the drop highlighter will turn red, but you will be able to do the drop nevertheless.