[common] indicate DASH container in _parse_mpd_formats function

This fixes issue #3681 and other similar for non-YouTube video sites
that use MPEG-DASH.

Signed-off-by: Ondřej Caletka <ondrej@caletka.cz>

youtube_dl/extractor/common.py

@@ -1879,6 +1879,7 @@ class InfoExtractor(object):
                             'language': lang if lang not in ('mul', 'und', 'zxx', 'mis') else None,
                             'format_note': 'DASH %s' % content_type,
                             'filesize': filesize,
+                            'container': mimetype2ext(mime_type) + '_dash',
                         }
                         f.update(parse_codecs(representation_attrib.get('codecs')))
                         representation_ms_info = extract_multisegment_info(representation, adaption_set_ms_info)