diff --git a/test/test_utils.py b/test/test_utils.py
index aedb565ec..7a507b9c3 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -315,6 +315,7 @@ def test_prepend_extension(self):
         self.assertEqual(prepend_extension('abc', 'temp'), 'abc.temp')
         self.assertEqual(prepend_extension('.abc', 'temp'), '.abc.temp')
         self.assertEqual(prepend_extension('.abc.ext', 'temp'), '.abc.temp.ext')
+        self.assertEqual(prepend_extension('..ext', 'temp', 'temp', True), '..ext.temp.temp')
 
         # Test uncommon extensions
         self.assertEqual(prepend_extension('abc.ext', 'bin'), 'abc.bin.ext')
diff --git a/yt_dlp/YoutubeDL.py b/yt_dlp/YoutubeDL.py
index 67ca90349..ce791eee7 100644
--- a/yt_dlp/YoutubeDL.py
+++ b/yt_dlp/YoutubeDL.py
@@ -3406,7 +3406,8 @@ def existing_video_file(*filepaths):
                     file = self.existing_file(itertools.chain(*zip(map(converted, filepaths), filepaths)),
                                               default_overwrite=False)
                     if file:
-                        info_dict['ext'] = os.path.splitext(file)[1][1:]
+                        real_ext = os.path.splitext(file)[1][1:]
+                        info_dict['ext'] = real_ext if real_ext else info_dict['ext']
                     return file
 
                 fd, success = None, True
diff --git a/yt_dlp/postprocessor/ffmpeg.py b/yt_dlp/postprocessor/ffmpeg.py
index 59a49aa57..18d555043 100644
--- a/yt_dlp/postprocessor/ffmpeg.py
+++ b/yt_dlp/postprocessor/ffmpeg.py
@@ -706,7 +706,7 @@ def run(self, info):
             self.to_screen('There isn\'t any metadata to add')
             return [], info
 
-        temp_filename = prepend_extension(filename, 'temp')
+        temp_filename = prepend_extension(filename, 'temp', info['ext'], True)
         self.to_screen(f'Adding metadata to "{filename}"')
         self.run_ffmpeg_multiple_files(
             (filename, metadata_filename), temp_filename,
diff --git a/yt_dlp/utils/_utils.py b/yt_dlp/utils/_utils.py
index 20aa341ca..5a05e9d9c 100644
--- a/yt_dlp/utils/_utils.py
+++ b/yt_dlp/utils/_utils.py
@@ -2122,7 +2122,7 @@ def parse_duration(s):
         (days, 86400), (hours, 3600), (mins, 60), (secs, 1), (ms, 1)))
 
 
-def _change_extension(prepend, filename, ext, expected_real_ext=None):
+def _change_extension(prepend, filename, ext, expected_real_ext=None, must_have_ext=False):
     name, real_ext = os.path.splitext(filename)
 
     if not expected_real_ext or real_ext[1:] == expected_real_ext:
@@ -2131,6 +2131,11 @@ def _change_extension(prepend, filename, ext, expected_real_ext=None):
             _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
             return f'{filename}.{ext}{real_ext}'
 
+    if not real_ext and must_have_ext and expected_real_ext and prepend:
+        _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
+        _UnsafeExtensionError.sanitize_extension(expected_real_ext)
+        return f'{filename}.{ext}.{expected_real_ext}'
+
     return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'