download helper not saving video files with video extension such as '.mp4' - only saving as a 'file'

I've downloaded the Firefox add-on "DownloadHelper" but no matter which option I try to save a video as (mp4 or flv etc) when I go to the file in my computer, it has not saved with an extension of any sort and is just saved as 'file'. I installed ConvertHelper and tried converting the videos, but it still only saves them as 'file'. Any help much appreciated.