Support attachments at whole-map level
Occasionally I need to attach a file to a map at the map level, not at the topic level. The nearest option is the Central Topic, which is less than ideal and some types of map (such as concept maps or flowcharts) don't have a central topic anyway. Alternatively a dedicated floating topic can be used, but you have to locate it first.
It would be useful to attach files to the map background, using an Attachments submenu in the canvas context menu. It could be shown in the Map Elements tab in the Index task pane.