[Zope] Image Upload

Hi there, I'm trying to insert an image in Zope by uploading it in an HTML 'file' field, using the following validation code: <dtml-if "banner_1 <> _.None"> <dtml-call "banners.manage_addImage(id='',file=banner_1)"> </dtml-if> When I submit an empty 'file' field, the dtml-call is invoked just the same, and I get an error message saying that the field is empty; when I check what's in REQUEST I see this: banner_1 <ZPublisher.HTTPRequest.FileUpload instance at 85fb998> Therefore, I reckon there must be something wrong with this validation routine. What should I do in order to run it properly? Thanks,