avformat/matroskadec: Add webm file extension

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskadec.c

@@ -4317,7 +4317,7 @@ static const AVClass webm_dash_class = {
 AVInputFormat ff_matroska_demuxer = {
     .name           = "matroska,webm",
     .long_name      = NULL_IF_CONFIG_SMALL("Matroska / WebM"),
-    .extensions     = "mkv,mk3d,mka,mks",
+    .extensions     = "mkv,mk3d,mka,mks,webm",
     .priv_data_size = sizeof(MatroskaDemuxContext),
     .read_probe     = matroska_probe,
     .read_header    = matroska_read_header,