Lowercase the parsed options names

I cannot find a RFC that indicates that the option names are case
sensitive whereas RFC6266 states that the `filename` option name is
case insensitive. Therefore the best user experience is to lowercase
the names.

This will cause breaking changes if case sensitivity is relied upon,
and hence users will need to use the lowercase name if so.

CHANGES.rst

@@ -9,7 +9,9 @@ Version 2.2.0
 -   Fix compatibility with Python 3.11 by ensuring that ``end_lineno``
     and ``end_col_offset`` are present on AST nodes. :issue:`2425`
 -   Add a new faster matching router based on a state
-    machine. :pr:`2433`.
+    machine. :pr:`2433`
+-   Names within options headers are always converted to lowercase. This
+    matches :rfc:`6266` that the case is not relevant. :issue:`2442`
 
 
 Version 2.1.2

src/werkzeug/http.py

@@ -390,6 +390,9 @@ def parse_options_header(
 
     :param value: The header value to parse.
 
+    .. versionchanged:: 2.2
+        Option names are always converted to lowercase.
+
     .. versionchanged:: 2.1
         The ``multiple`` parameter is deprecated and will be removed in
         Werkzeug 2.2.
@@ -440,7 +443,7 @@ def parse_options_header(
                 if not encoding:
                     encoding = continued_encoding
                 continued_encoding = encoding
-            option = unquote_header_value(option)
+            option = unquote_header_value(option).lower()
 
             if option_value is not None:
                 option_value = unquote_header_value(option_value, option == "filename")

tests/test_http.py

@@ -358,6 +358,10 @@ def test_parse_options_header_broken_values(self):
         assert http.parse_options_header(" , a ") == ("", {})
         assert http.parse_options_header(" ; a ") == ("", {})
 
+    def test_parse_options_header_case_insensitive(self):
+        _, options = http.parse_options_header(r'something; fileName="File.ext"')
+        assert options["filename"] == "File.ext"
+
     def test_dump_options_header(self):
         assert http.dump_options_header("foo", {"bar": 42}) == "foo; bar=42"
         assert http.dump_options_header("foo", {"bar": 42, "fizz": None}) in (