Rob Allen has posted an example (a simple one to get you started) of creating a file upload form in the Zend Framework.

Zend Framework 1.5's Zend_Form component is missing support for the file input element as it is waiting on a file upload component to build upon. We're busy people, so we'll fake it...

His example gives a screenshot of the end result and includes all of the code needed to make it all work - the form, the custom file form element, a ValidFile validation class to ensure you're getting exactly what you want and the controller to define the form and execute it once the user submits.