Fix guess_type defalut behavior

Author: rruuaanng

Lib/mimetypes.py

@@ -90,6 +90,8 @@ def add_type(self, type, ext, strict=True):
         list of standard types, else to the list of non-standard
         types.
         """
+        if not type and ext in self.types_map[strict]:
+            type = self.types_map[strict][ext]
         self.types_map[strict][ext] = type
         exts = self.types_map_inv[strict].setdefault(type, [])
         if ext not in exts:

Misc/NEWS.d/next/Library/2025-01-15-12-04-30.gh-issue-128703.6WPf38.rst

@@ -0,0 +1,2 @@
+Fix :meth:`mimetypes.MimeTypes.guess_type` to use default mapping for empty
+``Content-Type`` in registry.