[Zope] FTP upload of ExtImage

Hi there! Can anyone explain me why the following doesn't work: I'm trying to upload ExtImage to new folder in Zope using FTP. I have found that to tell Zope to create ExtImage instead of Image I should write external method PUT_factory and put it into the folder where I'm going to store image. For uploading ExtImage I use storbinary method of FTP object (file is representing as StringIO object). BUT: My PUT_factory doesn't work! Who can explain why? What am I doing wrong? Thanks, Alexander Klimovich