Xsendfile

XSendFile is an Apache module

http://ubuntuforums.org/showthread.php?t=1019214

as root

apt-get install libapache2-mod-xsendfile
a2enmod xsendfile
/etc/init.d/apache restart

In your virtual host file, you need the following:

XSendFile on
XSendFileAllowAbove on