You could start with this :
add_action( 'comment_form_logged_in_after', 'wpse_102612_additional_fields' );// only for logged users
function wpse_102612_additional_fields () {
echo '<p class="comment-form-upload">'.
'<label for="upload">' . __( 'Upload your file' ) . '</label>'.
'<input id="upload" name="upload" type="file" size="30" /></p>';
}