diff --git a/youtube_dl/extractor/canalc2.py b/youtube_dl/extractor/canalc2.py
index d9137e2efc..ba82bb2b73 100644
--- a/youtube_dl/extractor/canalc2.py
+++ b/youtube_dl/extractor/canalc2.py
@@ -34,6 +34,7 @@ def _real_extract(self, url):
             rtmp = re.search(r'^(?P<url>rtmp://[^/]+/(?P<app>.+/))(?P<play_path>mp4:.+)$', video_url)
             formats[0].update({
                 'url': rtmp.group('url'),
+                'ext': 'flv',
                 'app': rtmp.group('app'),
                 'play_path': rtmp.group('play_path'),
                 'page_url': url,