[utils] mimetype2ext: return 'm4a' for 'audio/mp4' (fixes #8620)

The youtube extractor was using 'mp4' for them, therefore filters like 'bestaudio[ext=m4a]' stopped working (94278f720272c5ad2cd5900f59f8e71f31d46633 broke it).

youtube_dl/utils.py

@@ -1836,6 +1836,12 @@ def error_to_compat_str(err):
 
 
 def mimetype2ext(mt):
+    ext = {
+        'audio/mp4': 'm4a',
+    }.get(mt)
+    if ext is not None:
+        return ext
+
     _, _, res = mt.rpartition('/')
 
     return {