Steps to Reproduce

trigger a download with a specific charset

     byte[] data = "Price 500,00 €".getBytes("ISO-8859-15");
     Filedownload.save(data, "text/plain;charset=ISO-8859-15", "mytext.txt");

or a binary download without a character encoding

     Filedownload.save("mysheet.xls", "application/vnd.ms-excel");

Current Result

the response header appends ;charset=utf-8 unnecessarily for binary files or overrides the provided values

Content-Type: text/plain;charset=utf-8

Content-Type: application/vnd.ms-excel;charset=utf-8

Expected Result

Content-Type: text/plain;charset=ISO-8859-15

Content-Type: application/vnd.ms-excel

Debug Information

for each response the character encoding is set to utf-8

https://github.com/zkoss/zk/blob/v9.0.0-RC2/zk/src/org/zkoss/zk/au/http/DHtmlUpdateServlet.java#L453