imposible to say what the problem is without doing some troubleshooting.
I would create a new blank page, add a file field to it and add the file upload behavior for a test.
set up the upload on this page exactly the same as on the one that doesn't work.
after adding the file upload behavior to it, add the following code at line 1:
next go to the bindings panel and expand the WA UPloaded Files Global Status group and add the debug info binding to your page.
also add this code just before the </body> tag:
<pre><?php var_dump($_POST); ?></pre>
<pre><?php var_dump($_FILES); ?></pre>
upload that to your server and test the file upload to see if it works.