[PR #10880/3c9d7abf backport][3.11] Add invalid content type test docs

docs/client_reference.rst

@@ -1455,7 +1455,12 @@ Response object
 
          Returns value is ``'application/octet-stream'`` if no
          Content-Type header present in HTTP headers according to
-         :rfc:`2616`. To make sure Content-Type header is not present in
+         :rfc:`9110`. If the *Content-Type* header is invalid (e.g., ``jpg``
+         instead of ``image/jpeg``), the value is ``text/plain`` by default
+         according to :rfc:`2045`. To see the original header check
+         ``resp.headers['CONTENT-TYPE']``.
+
+         To make sure Content-Type header is not present in
          the server reply, use :attr:`headers` or :attr:`raw_headers`, e.g.
          ``'CONTENT-TYPE' not in resp.headers``.
 

tests/test_web_response.py

@@ -1122,6 +1122,13 @@ def test_ctor_content_type_with_extra() -> None:
     assert resp.headers["content-type"] == "text/plain; version=0.0.4; charset=utf-8"
 
 
+def test_invalid_content_type_parses_to_text_plain() -> None:
+    resp = Response(text="test test", content_type="jpeg")
+
+    assert resp.content_type == "text/plain"
+    assert resp.headers["content-type"] == "jpeg; charset=utf-8"
+
+
 def test_ctor_both_content_type_param_and_header_with_text() -> None:
     with pytest.raises(ValueError):
         Response(