archive module: add sort feature to achive files

app/models/mongoid/sortable.rb

@@ -21,7 +21,7 @@ module Mongoid
     private
 
     def set_sort_number
-      if self.sort_number.nil?
+      if self.new? && self.sort_number.nil?
         last_record = self.class.desc(:sort_number).first
         if last_record
           self.sort_number = (last_record.sort_number + 10) / 10 * 10