diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index 7ce661b09..a2c6780ca 100644
--- a/youtube_dl/utils.py
+++ b/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 {