file_tag and full file path

I am using a file_field_tag and when I am getting the value on the controller, it only gives me the file name and not the file path. I really need the full file path, how can i do it ? I'll give you a part of my code.

<form action= "/entrees/lireXML" > <label>Fichier:</label><%= file_field_tag "xml_file" %>     <%= submit_tag "Envoyer" %> </form>

def lireXML   load_file = params[:xml_file] remplieBD ( load_file )     redirect_to (entrees_url)   end

actually i found out that it works with internet explorer but not with firefox ! Anyone know how to manage to make it work on firefox please ?