I am sorry, I can't be more specific, but there since the uploading mechanism changed in 1.4.5, we have been experiencing very strange problems with Rails (actually the Ruby CGI handling and Lighttpd). There is no problem in 1.4.4 and before.
This is the error that Ruby throws with 1.4.7:
Dispatcher failed to catch: Connection reset by peer (Errno::ECONNRESET)
/web/rails/app/current/public/../config/../vendor/rails/actionpack/lib/action_controller/cgi_process.rb:148:in `write'
/web/rails/app/current/public/../config/../vendor/rails/actionpack/lib/action_controller/cgi_process.rb:148:in `out'
/web/rails/app/current/public/../config/../vendor/rails/railties/lib/dispatcher.rb:34:in `dispatch'
/web/rails/app/current/public/../config/../vendor/rails/railties/lib/fcgi_handler.rb:144:in `process_request'
/web/rails/app/current/public/../config/../vendor/rails/railties/lib/fcgi_handler.rb:64:in `process!'
/web/rails/app/current/public/../config/../vendor/rails/railties/lib/fcgi_handler.rb:55:in `each_cgi'
/usr/lib/ruby/site_ruby/1.8/fcgi.rb:597:in `each'
/usr/lib/ruby/site_ruby/1.8/fcgi.rb:597:in `each_cgi'
/web/rails/app/current/public/../config/../vendor/rails/railties/lib/fcgi_handler.rb:55:in `process!'
/web/rails/app/current/public/../config/../vendor/rails/railties/lib/fcgi_handler.rb:21:in `process!'
/web/rails/app/current/public/dispatch.fcgi:24
