MimeType definition not applied to outgoing file

I added the following line to mime_types.rb: Mime::Type.register "application/x-xpinstall", :xpi in order to associate the content-type "application/x-xpinstall" with a Firefox extension "xxx.xpi" (and I restarted the local server). However, when key in the URL or click a link to the file, nothing appears in the server log and the file is downloaded apparently as "application/octet-stream". Just in case, I also tried #register_alias, which also did not work.