[Zope] determining if there is a file to be uploaded

Hi! I have a form with a file-input tag. Now, how do I determine if there is actually a file upload done? (I tried to read the file size with <dtml-if "_.len(file.read())"> there is data <dtml-else> there is no data </dtml-if> But this does not work (always 0). Ok, how is this to be done? Regards, Ingo.