Archive shows metadata = doc

Sample output from the button code snippet appended below:

f = C:\Users\admin\Downloads\Beach and Sand.zip
md = doc, grps = Archives
f.metadata.tags = just;testing

This is consistent for all .zip files that I have tried. I would expect metadata to be "none" rather than "doc".

function OnClick(clickData)
{
	if (clickData.func.sourcetab.selected_files.count==0) return;
	DOpus.ClearOutput();
	var f = clickData.func.sourcetab.selected_files(0);
	DOpus.output("f = "+f);
	var md = f.metadata;
	if (String(md)=="none") {
		DOpus.output("md = "+md);
		return;
	}
	var grps;
	for (var i = 0; i < f.groups.count; i++) grps = (i==0) ? f.groups(i) : grps+", "+f.groups(i);
	DOpus.output("md = "+md+", grps = "+grps);
	DOpus.output("f.metadata.tags = "+stringTags(f.metadata.tags));
}
function stringTags(v)
{
	var str = "";
	if (typeof v!="object") return v;
	if (typeof v.count!="number") return v;
	for (var i = 0; i < v.count; i++) str = (i>0) ? str+";"+v(i) : v(i);
	return str; 
}

Archives contain the fields metadata.doc.doccreateddate and metadata.doc.doclastsaveddate. That probably sets their primary metadata type to "doc".