fix(filefield): php warning when editing the question

Signed-off-by: Thierry Bugier <tbugier@teclib.com>

Author: btry

inc/fields/filefield.class.php

@@ -109,6 +109,10 @@ public static function getName() {
       return __('File');
    }
 
+   public function prepareQuestionInputForSave($input) {
+      return $input;
+   }
+
    public static function canRequire() {
       return true;
    }