You can accomplish this with a routing rule like the following:
route '*' do
item.identifier.chop + '.' + item[:extension]
end
If your goal is to have an area under content/
that gets copied verbatim to the output, a better approach would be to set up a static data source that will read these files without dropping their extension and also allow you to keep them in a folder separate from the other, non-static items. The nanoc troubleshooting page has instructions on setting this up ("Solution #2").