var context = SP.ClientContext.get_current();
var web = context.get_web();
var items = SP.ListOperation.Selection.getSelectedItems(context);
var currentLibrary = web.get_lists().getById(SP.ListOperation.Selection.getSelectedList(context));
var index;
for (index in items) {
    var selectedItem = currentLibrary.getItemById(items[index].id);
    // Need to identify if 'selectedItem' is folder or not
    console.log("FileSystemObjectType = " + selectedItem.FileSystemObjectType); // Returns 'undefined'
    console.log("get_file = " + selectedItem.get_file()); // Returns 'SP.ListItem' object
    console.log("get_folder = " + selectedItem.get_folder()); // Returns 'SP.ListItem' object
}

if(item.FileSystemObjectType == SP.FileSystemObjectType.Folder) {
    // Always returns true
}