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.

Fichier:<%= file_field_tag "xml_file" %> <%= submit_tag "Envoyer" %>

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

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 ?