[Zope] File upload

Learning Zope is sometimes not really funny! Ok, here's another question of mine: In a form I give users the possibility to upload a file with the following tag: <input type=file name="file"> In the next dtml method which is called from here I want to test, if the user has selected a file or not. If the input field is empty, I don't want to do anything, but how could i test? I tried it with: <dtml-if REQUEST.form['file']<>''> ... but without succes! Perhaps some can explain HOW and WHY, please!!! Best Regards, ... Marc