[Zope] Image Upload

[Fabio da Silva Santos]

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,