Configuration for Lighttpd fastcgi-php.conf
When using Lighttpd as the server technology, it is necessary to ensure that your fastcgi-php.conf file is configured correctly, otherwise you may experience problems with file downloads.
In your fastcgi-php.conf file, ensure you have allow-x-send-file enabled, like so:
"allow-x-send-file" => "enable"
If you do not enable allow-x-send-file, you may get corrupted file downloads, as detailed in this support ticket.