delete apex static files

In lower versions of apex(my 4.1), there is bug for deleting static files from server, so you must delete file through DB. This is how :

   declare  
    v_sec_grp_id varchar2(2000);  
   begin  
   -- get security group id  
   select to_char(workspace_id) sec_group_id into v_sec_grp_id from apex_workspaces w;  
   -- set security group id  
   apex_util.set_security_group_id(v_sec_grp_id); 
   -- delete static file  
   delete from wwv_flow_files where filename = 'custom.css';  
   end;