Function write_file_dnd_messages_system 

pub fn write_file_dnd_messages_system(
    dnd_reader: EguiContextMessageReader<'_, '_, FileDragAndDrop>,
    egui_file_dnd_message_writer: MessageWriter<'_, EguiFileDragAndDropMessage>,
    egui_contexts: Query<'_, '_, &EguiContextSettings, With<EguiContext>>,
)

Reads FileDragAndDrop messages and wraps them into EguiFileDragAndDropMessage, can redirect messages to HoveredNonWindowEguiContext.