thumbnails are not generated in backend [fixed]

[carin]

reproduce:
* upload images to the root jd folder via ftp
* back end login
* add a new downlod and choose an existing file on the server previously uploaded

expected:
* thumbnails are generated and linked correctly
* description is set correctly

current:
* <img src="/" alt="Materialbibliothek - {description_long}"> (link is wrong, description is wrong {description_long})