My app is a surveybuilder...it needs upload cvs files by users, Im working with rails 3.1.3., ruby 1.9.2 ande devise 1.5.3 for authentication, but I tried: http://www.jedi.
Carrierwave (https://github.com/jnicklas/carrierwave) is pretty much the standard when it comes to files uploading.
Otherwise, here is a simplier method if you don't need a full-fledged gem: Rails 3 - upload files to public directory