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 ?