[FFmpeg-user] can't concatenate the flv file

[Liam Condron-Farnos]

The flv container format doesn't support file-level concatenation; only a
few container file types do. So the concat protocol won't work on them.

The concat demuxer, as its name suggests, demuxes the input streams from
their container and then concatenates them, so it doesn't care about what
the input container is (and if the codecs are all the same w/ similar
characteristics, you can actually concatenate files of different container
formats).