You can't output binary data of any kind in a portlet. All portlet
output must be the primary output type for the device you are using.
For example, text/html.

To accomplish what you want to do, you will need to either put the excel
data in a file on the filesystem or serve it up through a servlet either
packaged with your portlet or somewhere else on your server.

Then, in the portlet output, write markup that causes the browser to
access the servlet/file to display the data. For example, in an