diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index d4654cc05..b09eeff32 100644
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -484,6 +484,9 @@ class YoutubeDL(object):
                     res=self.format_resolution(format),
                     note=u' ({})'.format(format['format_note']) if format.get('format_note') is not None else '',
                 )
+            # Automatically determine file extension if missing
+            if 'ext' not in format:
+                format['ext'] = determine_ext(format['url'])
 
         if self.params.get('listformats', None):
             self.list_formats(info_dict)
diff --git a/youtube_dl/extractor/addanime.py b/youtube_dl/extractor/addanime.py
index 3b8258ad8..b99d4b966 100644
--- a/youtube_dl/extractor/addanime.py
+++ b/youtube_dl/extractor/addanime.py
@@ -71,7 +71,6 @@ class AddAnimeIE(InfoExtractor):
             formats.append({
                 'format_id': format_id,
                 'url': video_url,
-                'ext': video_url[-3:],
             })
         if not formats:
             raise ExtractorError(u'Cannot find any video format!')