[Zope] verify whether html <input type=file... is empty

Hi! Don't know if it is really a zope question, but I have a problem, verifying if e.g. input <INPUT TYPE="file" NAME="image_1" SIZE="50"> is empty or not using <dtml-unless " REQUEST[ 'image_1']=='' "> <dtml-call "manage_addImage( id=imageId, file=REQUEST[ image_1], REQUEST=REQUEST )"> </dtml-unless> well, I think the <input type=file... is not a string, but what I want to test is if the user put in the filename and not whether the really existing file is empty, so how could aI do it? I would appreciate any hints Thank you in advance Andrey