The user would have to have an application associated with that file type for the browser to know what to do with it.
If they do, you can set the Content-Type header with the appropriate mime type. Then set the Content-Disposition header using 'inline' instead of 'attachment'.
If all these things are done, the browser should open the file in the associated application.
See the JspAndExcel entry in the
Servlets Faq.
-Ben
PS: If you're working with a binary Excel file, you'll probably need to stream the data to the browser using a servlet (JSPs don't do this well).
In our
CodeBarn there is a servlet example called SimpleStream that shows how to do this.