Tomcat: form-based authentication problem changing content type of a JSP

When I open a new browser in a form-based authentication application
this new browser window is not authenticated as the opener window
is...

I have set the content type of the document that I open in this new
browser window to application/vnd.ms-excel...

Can you tell me if that's a normal behaviour and if it's possible to
avoid this? (I mean, if I'm going to be able to open this new browser
window, with the application/vnd.ms-excel document, without having to
stablish the username and password again)

Thanks in advance!

Raul (Spain)