So I was wondering I kept having to re-select 'flash actionscript' on the languages menu every time I opened a file for editing to apply the custom formatting I'd created.
Both the Haskell language and the Flash Actionscript language have "as" defined as a default extension and NPP++ was defaulting to Haskell.
This can't be changed through the GUI. You have to manually edit the configuration file yourself. This should either be documented or have a button added to pick which language the extension "as" should apply to.