On the Zend Developer Zone there's a post looking at uploading files in a Adobe Flex-based application that uses a slightly different than usual approach to uploading files through Zend_Amf.

Zend AMF is a PHP implementation of the AMF (Action Message Format) binary protocol within the Zend Framework. I had to implement a system to upload files that were a little different than what is typically used in Flash, so I built upon Zend AMF for my needs. Researching a little on the net, I found a solution that was simpler than I expected. Based on an article, I only had to make a few adjustments.

He shows a (brief) version of his setup - a gateway to be used as the endpoint for Flex to connect to, the VO properties and the class to handle the upload. On the Flex side, he includes the code to create the view and pass the upload off to the waiting PHP backend. If you want to see the full code, you can grab it here.