org.zkoss.zkmax.zul.FiledownloadListener.class
From Documentation
Default: none [Enterprise Edition] [Since 3.5.0]
It specifies the name of the class that will be notified when a file download is happening. The class must implement the FiledownloadListener interface.
A developer usually uses it to reject a download if the user tried to resume it in an unacceptable situation. For example, a developer can limit the user from downloading within upcoming 24 hours.