Resolve #1426 "Dateiverwaltung: Leere Dateinamen beim Herunterladen"

Closes #1426

Merge request studip/studip!910

lib/models/FileRef.php

@@ -143,7 +143,7 @@ class FileRef extends SimpleORMap implements PrivacyObject, FeedbackRange
         $link = [];
         $params = [];
         $type = '0';
-        $file_name = $this->file->name;
+        $file_name = $this->file->name ?: $this->name;
         $file_id = $this->id;
 
         switch($mode) {