[Zope] File upload

[Marc Fischer]

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