diff --git a/youtube_dl/PostProcessor.py b/youtube_dl/PostProcessor.py
index 69aedf87a4..097e1a9e41 100644
--- a/youtube_dl/PostProcessor.py
+++ b/youtube_dl/PostProcessor.py
@@ -10,6 +10,7 @@
     PostProcessingError,
     shell_quote,
     subtitles_filename,
+    prepend_extension,
 )
 
 
@@ -496,13 +497,11 @@ def run(self, info):
             return True, info
 
         filename = info['filepath']
-        ext = os.path.splitext(filename)[1][1:]
-        temp_filename = filename + u'.temp'
+        temp_filename = prepend_extension(filename, 'temp')
 
         options = ['-c', 'copy']
         for (name, value) in metadata.items():
             options.extend(['-metadata', '%s=%s' % (name, value)])
-        options.extend(['-f', ext])
 
         self._downloader.to_screen(u'[ffmpeg] Adding metadata to \'%s\'' % filename)
         self.run_ffmpeg(filename, temp_filename, options)
diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index da5143c8ef..fc10fba632 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -1119,3 +1119,8 @@ def parse_duration(s):
         if m.group('hours'):
             res += int(m.group('hours')) * 60 * 60
     return res
+
+
+def prepend_extension(filename, ext):
+    name, real_ext = os.path.splitext(filename) 
+    return u'{0}.{1}{2}'.format(name, ext, real_ext)