[ExtractAudio] Handle outtmpl without ext (#6005)

Authored by: carusocr
Closes #5968

yt_dlp/__init__.py

@@ -318,10 +318,6 @@ def validate_options(opts):
     if outtmpl_default == '':
         opts.skip_download = None
         del opts.outtmpl['default']
-    if outtmpl_default and not os.path.splitext(outtmpl_default)[1] and opts.extractaudio:
-        raise ValueError(
-            'Cannot download a video and extract audio into the same file! '
-            f'Use "{outtmpl_default}.%(ext)s" instead of "{outtmpl_default}" as the output template')
 
     def parse_chapters(name, value):
         chapters, ranges = [], []

yt_dlp/postprocessor/ffmpeg.py

@@ -508,8 +508,7 @@ class FFmpegExtractAudioPP(FFmpegPostProcessor):
         if acodec != 'copy':
             more_opts = self._quality_args(acodec)
 
-        # not os.path.splitext, since the latter does not work on unicode in all setups
-        temp_path = new_path = f'{path.rpartition(".")[0]}.{extension}'
+        temp_path = new_path = replace_extension(path, extension, information['ext'])
 
         if new_path == path:
             if acodec == 'copy':