With Bug 1800972 fixed Thunderbird can drop multiple attachments to desktop or file manager.

But when the attachments are dragged, only one attachment (the one under mouse cursor) is show so it looks like only one is transferred.

Thanks Martin! That's true. It's confusing to see only one item as drag image while dragging multiple items. The drag image should represent multiple items somehow, even adding multiple shadows to the drag image, or an overlay icon with the number of dragged items on the drag image.

Alex, any insights what's needed or desirable here? Is it hard to fix?

I'm not sure. Magnus might be more suitable to identify what's needed for TB to leverage the m-c code.

Looks like DataTransfer.setDragImage should be invoked, which should be easy enough.
But we'd need to generate something to show (pass to that). Not sure what that should look like.
I assume Firefox doesn't do anything automatically here?

If I open local folder in firefox:

file:///home/user

it shows listed files. When D&D is performed on more selected files, a D&D icon contains all files. That's all I know.