use data-file to retrieve filename

forward port of https://github.com/owncloud/core/pull/5647#discussion-diff-7341163
trust me, I know what I'm doing

apps/files/js/files.js

@@ -487,7 +487,7 @@ var folderDropOptions={
 			return false;
 		}
 
-		var target = $.trim($(this).find('.nametext').text());
+		var target = $(this).parent('tr').data('file');
 
 		var files = ui.helper.find('tr');
 		$(files).each(function(i,row) {