respond_to, mime types and excel

Hello All,

I'm trying to figure out how respond_to actually works, especially with
the increased focus on REST.

After I set a MIME type for Excel

Mime::Type.register "application/", :xls

In my index action's respond_to block

I have format.xls {
@parts = Part.find(:all)
        headers['Content-Type'] = "application/"
        headers['Content-Disposition'] = "attachment;
        headers['Cache-Control'] = ''
        render( :action=>"inbound.rxml", :layout => false)

but how do i actually trigger the action to return the spreadsheet
instead of the HTML page?

when i try index.xls it tries to render the show function.

If anyone can explain this that would be much appreciated.