This is a classic case of ie implementing different standards in javascript than other browsers.
to pull IE out of the equation, you should validate the file field server side instead of using client side validation.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.