I was struggling with this for a bit. But luckily there is a quick work around. It seems the scope is lost for the event listener when using flash.system.File browseForOpen(), i.e. your handler will never know when a file is selected. The solution is to move the variable outside of the function - create an instance variable of File.

Read this thread for more info:
http://www.adobe.com/cfusion/webforums/forum/messageview.cfm?catid=641&threadid=1275