Property:

org.zkoss.zul.Filedownload.contentTypeAsIs

Default: false

It specifies whether to treat the Content-Type as is in Filedownload class.

By default, ZK will append ;charset=UTF-8 in the Content-Type to ensure everything is displayed in UTF-8. But in some cases, you don’t want ZK to add this in a binary file or you want to specify another charset. Enable this property and ZK will respect what you set in the Content-Type and use it as is.

See: ZK-4418