data FileFormat
  = Spreadsheet
  | Picture
  | Video
  deriving Show

data TmpFile = TmpFile FileFormat FilePath
  deriving Show

videoPath :: TmpFile -> FilePath
videoPath (TmpFile Video p) = p
videoPath _ = error "only works on videos!"

type TmpSpreadsheet = TmpFile Spreadsheet
type TmpPicture     = TmpFile Picture
type TmpVideo       = TmpFile Video

videoPath :: TmpVideo -> FilePath

data TmpFile a = TmpFile a FilePath
  deriving Show

videoPath :: TmpFile Video -> FilePath

{-# LANGUAGE EmptyDataDecls #-}

data Spreadsheet
data Picture
data Video

newtype TmpFile a = TmpFile FilePath

videoPath :: TmpFile Video -> FilePath
videoPath (TmpFile p) = p

λ> let video = TmpFile "/video/lyah.avi" :: TmpFile Video

λ> videoPath video
"/video/lyah.avi"

λ> let picture = TmpFile "/video/lyah.png" :: TmpFile Picture

λ> videoPath picture
-- ERROR