In that case a Ruby on Rails mailing list may not be the best place to get help.
But in your situation the first thing I would try is to run it in
firebug in firefox to check for errors. Also paste the complete page
html into the w3c html validator to check for valid html. Differences
between browsers is often caused by invalid html.
To be honest, I'm surprised this works at all - file fields are
notoriously resistant to being read via JS, for security reasons. If
this is a critical part of your application, you may need to go to an
alternative method (Flash, for instance) to have it work reliably
across browsers and security settings.