Drupal form does not save the user input value in the $form_state array
File input fields work differently than other input fields. This is a restriction of HTML. The file path is not transmitted to the server, because it is considered sensitive information. Only the file data and the name of the file is transmitted to the server. 

Use file_save_upload() to save the file. To find out what to pass as the $source argument, print out both $form_state[values] as well as the $_FILES array.