diff --git a/src/calibre/__init__.py b/src/calibre/__init__.py
index 41c650fd88..37bb1bb635 100644
--- a/src/calibre/__init__.py
+++ b/src/calibre/__init__.py
@@ -56,6 +56,7 @@ def _init_mimetypes():
     _mt_inited = True
 
 
+@lru_cache(4096)
 def guess_type(*args, **kwargs):
     import mimetypes
     if not _mt_inited: