As always IE sails its own course... to fix the problem you need a couple of extra headers:
In the same file: environment/functions/files.php
Search for: header("Content-Transfer-Encoding: binary"); (there should be two occurrences)
After this line add the following two lines:
header("Cache-Control: maxage=1"); // Age is in seconds.
header("Pragma: public");
Voila, IE will download in HTTPS again.
Ignacio, if you could add these changes to the source code it would be extremely helpful for future releases. Currently downloads are borked in all patched versions of IE6, IE7, IE8 when accessed via HTTPS.
Thanks,
Rob.