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

@@ -6,6 +6,8 @@ Version 2.2.0
 -   Added the ``werkzeug.debug.preserve_context`` mechanism for
     restoring context-local data for a request when running code in the
     debug console. :pr:`2439`
+-   Names within options headers are now lowercased on parsing. This
+    matches RFC expectations that the casing is not relevant. :pr:`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 now lowercased.
+
     .. 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 (