video::video_file.mp4[poster="sunset.jpg"]
>>>
Document
  { docMeta =
      Meta
        { docTitle = []
        , docTitleAttributes = Nothing
        , docAuthors = []
        , docRevision = Nothing
        , docAttributes = fromList [ ( "sectids" , "" ) ]
        }
  , docBlocks =
      [ Block
          Attr
          ( [] , fromList [ ( "poster" , "sunset.jpg" ) ] )
          Nothing
          (BlockVideo (Target "video_file.mp4"))
      ]
  }