Function translate_file_drops_to_payload 

pub fn translate_file_drops_to_payload(
    file_drops: MessageReader<'_, '_, FileDropped>,
    accept: Query<'_, '_, &DropAccept>,
    out: MessageWriter<'_, DropAccepted>,
)

Translate inbound platform FileDropped events into DropAccepted when the target carries a DropAccept filter. Targets without a DropAccept keep the old “accept anything” behaviour, so apps/drop-target keeps working.

This is the inbound half, the drag the platform hands the window. The in-app half - a drag that starts on a DragSource in the same app - runs through begin_in_app_drag, track_drop_hover and finish_in_app_drag, which DndPlugin registers alongside this one.