title: ffmpeg concat error unable to find a suitable output format
body: <p>I'm trying to batch concat a outro video to multiple videos. 
  I am just tetsing it using the following command:
  <code>ffmpeg -i test.mp4 -i hello.mp4 -filter comp
  lex “[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]” -map “[v]” -map “[a]” outpu
  t_video.mp4
  </code></p>
  
  <blockquote>
    <p>However I get the following Error:</p>
  </blockquote>
  
  <p><code>[NULL @ 04fc2c00] Unable to find a suitable output format for '[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]'
  [0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]: Invalid argument</code></p>