[Koha] Upload files in cataloguing

Hi all I would like to upload a file and make it visible through the 856$u field. I followed the instructions on http://kohageek.blogspot.com/2015/12/attaching-files-to-catalog-records.html The plugin appears and I can upload the file. But when I select the Choose (or Auswählen) buttom nothing happens. The same with the Download and the "Löschen" buttom. If I click on koha_upload I'm able to download (and maybe delete - I haven't checked) the file. What is wrong? Thank you Beda