add missing imports

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@35288 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/mixin/base_convertable_and_file.py

@@ -28,6 +28,8 @@
 ##############################################################################
 from AccessControl import ClassSecurityInfo
 from Products.ERP5Type import Permissions
+from OFS.Image import Pdata
+import cStringIO
 
 _MARKER = []
 class BaseConvertableAndFileMixin: