foo.elm

type alias File = { name : String }

type Action = AgreeToTerms | AddFiles { files : List File }
type State = Onboarding | Form { files : List File }

action = AgreeToTerms
state = Form { files = [] }

result = case ( action, state ) of
  ( AgreeToTerms, Onboarding ) ->
    Form { files = [] }
  ( AddFiles { files }, Form form ) ->
    Form { files = form.files }
  ( _ , _ ) ->
    state