You can find the documentation here: http://help.adobe.com/en_US/adobemediaserver/ssaslr/WS5b3ccc516d4fbf351e63e3d11a11aff5ba-7f8bSSASLR.2.3.html
You are right, you can not use AS3 on server side applications.
You can use server-side Actionscript, wich is the equivalent to Javascript 1.5. FMS has a javascript engine that executes server-side scripts.
Also, you will notice that you can name the files either .asc or .js.