The import of filestream_iterator was removed

This commit:
https://github.com/zopefoundation/Zope/commit/ec703495f52ed0a52e566cf73a2c7f359ca387f3#src/OFS/Image.py
removes the import of "filestream_iterator".

It is also the last commit in branch 2.13 for Image.py:
https://github.com/zopefoundation/Zope/commits/2.13/src/OFS/Image.py

The same "Image.py" is also in the current version of Zope (2.13.19). 

My patch just adds the import again. Tested with Zope 2.13.19 and Plone 4.1.6