diff --git a/youtube_dl/extractor/dumpert.py b/youtube_dl/extractor/dumpert.py
index d9d9afdec..a73cd85a9 100644
--- a/youtube_dl/extractor/dumpert.py
+++ b/youtube_dl/extractor/dumpert.py
@@ -50,6 +50,7 @@ class DumpertIE(InfoExtractor):
                 'url': uri,
                 'format_id': version,
                 'quality': quality(version),
+                'ext': 'mp4',
             })
         self._sort_formats(formats)