[Zope] ExtImage and Email

Hello, I am trying to send an image of class ExtImage as attachment in an email. If image is of class Image I can do this: <dtml-boundary type=application/octet-stream disposition=attachment name=myTest.gif encode=base64> <dtml-var "myTest.data"> But which method (I couldn't find one) have I to use when image is of class ExtImage? Any hints? Best regards Arno