gh-130110: Support hyphens in RFC 2231 continuation parameter names

Parameter names containing hyphens (e.g. `file-name*0*`) were not
recognized as RFC 2231 continuations because the matching regular
expression only allowed `\w+`. As a result, such parameters were left
undecoded by `email.utils.decode_params`.

Adjust the regular expression to also accept hyphens (`[\w-]+`), which
matches the production behaviour for non-hyphenated names.

Fixes: gh-130110

Author: Phantom-d-e-v

Lib/email/utils.py

@@ -399,7 +399,7 @@ def encode_rfc2231(s, charset=None, language=None):
     return "%s'%s'%s" % (charset, language, s)
 
 
-rfc2231_continuation = re.compile(r'^(?P<name>\w+)\*((?P<num>[0-9]+)\*?)?$',
+rfc2231_continuation = re.compile(r'^(?P<name>[\w-]+)\*((?P<num>[0-9]+)\*?)?$',
     re.ASCII)
 
 def decode_params(params):

Lib/test/test_email/test_email.py

@@ -400,6 +400,18 @@ def test_continuation_sorting_part_order(self):
         filename = msg.get_filename()
         self.assertEqual(filename, 'foo bar.txt')
 
+    def test_continuation_with_hyphenated_name(self):
+        # gh-130110: parameter names containing hyphens were not recognized
+        # as RFC 2231 continuations and were left undecoded.
+        msg = email.message_from_string(
+            "Content-Disposition: attachment; "
+            "file-name*0*=\"utf-8''start\"; "
+            "file-name*1*=\"-middle-\"; "
+            "file-name*2*=\"end\"\n"
+        )
+        value = msg.get_param('file-name', header='content-disposition')
+        self.assertEqual(value, ('utf-8', '', 'start-middle-end'))
+
     def test_sorting_no_continuations(self):
         msg = email.message_from_string(
             "Content-Disposition: attachment; "