I would've went with Haskell myself ;)

    main :: IO ()
    main = writeListAsLines . toDoubleList . lines =<< readFile "nums.txt"
           where
             toDoubleList y = zipWith (*) (map read y) [1..length y]
             writeListAsLines z =  writeFile "output.txt" (concat $ intersperse "\n" $ map show z)