Information on syntax highlighting is here:
http://trac.edgewall.org/wiki/TracSyntaxColoring
I believe that ActionScript is based on ECMAScript a.k.a JavaScript, so
you could map the file extension to that mimetype in trac.ini with:
[mimeviewer]
application/x-javascript:fileextension
I don't know the proper file extension for ActionScript files, so
replace "fileextension" with whatever that should be.
-- Matt Good