Hi Stefan,
Welcome to The HUB!
The latest ExtremeWare software available for it is v7.8.4b1-patch1-4.
Just for reference, the Summit 48si product line will go End-of-Life on 6/30/2015.
I see that the support contract on that switch has expired (based on serial number). GTAC will be able to provide you with some options for obtaining the software.
-Drew