It wasn’t a .htaccess problem, but an apache configuration problem.
On a Mac server you need to:
- compile XSendFile as a universal binary:
apxs -cia -Wc,"-arch i386 -arch x86_64" -Wl,"-arch i386 -arch x86_64" mod_xsendfile.cand - modify the site specific httpd.conf file in
/Library/Server/Web/Config/apache2/sites.
If you’re having the same problem, I wrote about this on my blog – hopefully it’s helpful to someone!