It seems that its only possible to upload files over FTP using:
https://wiki.jenkins-ci.org/display/JENKINS/Publish+Over+FTP+Plugin
But I need to download a
Maybe the reason is that it is somewhat trivial to download a file during a build using a one-liner like:
curl -O ftp://server/path/to/file
The curl tool is available for just about any operating system.
If you only need to download one file over FTP and do not need to check out any other files, you could also use the URL SCM plugin.