I can't add anything to my previous response. You can't do this within the XSLT language. You will have to call out to some other technology, and the way you do it depends on your environment. You need (a) to convert the base64 to a stream of bytes or octets, (b) to write those bytes to a file, and (c) to generate in your HTML a link that points to that file. None of that is very difficult, but it's outside the scope of what XSLT can do, even in 2.0.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference