[Zope] Zope Product - zipfile not compressing

Hello folks, I wrote a custom zope product that I wanted to use create a zip archive from some of the files on the filesystem and then send this zipfile as a download. The product works, the zipfile gets created and all the files are there, only there appears to be no compression at all. Everything is just stored. Here is how I create the archive: zip = zipfile.ZipFile(file_name, mode='w', compression=zipfile.ZIP_DEFLATED) then I keep adding with writestr: zip.writestr(zinfo, obj_data) any thoughts? thanks and best regards. Istvan.