From 08ef4617e7034cbb1d967fa41e8507c759653887 Mon Sep 17 00:00:00 2001
From: JSap0914 <JSap0914@users.noreply.github.com>
Date: Tue, 30 Jun 2026 13:07:38 +0900
Subject: [PATCH] Fix parse_mimetype ignoring whitespace-only segments after
 semicolons

When a MIME type string contains a trailing semicolon followed by
whitespace (e.g. 'text/html; ' or 'text/html; charset=utf-8; '),
the whitespace-only segment was not being skipped, resulting in a
spurious empty-key parameter ({"": ""}) being added to the
parsed MimeType's parameters dict.

The existing check 'if not item: continue' only skips truly empty
strings (from a trailing bare semicolon like 'text/html;'), but
not whitespace-only strings produced by 'text/html; '.

Fix: use 'if not item.strip(): continue' to also skip whitespace-only
segments, consistent with RFC 2045 which requires parameter names
to be non-empty tokens.
---
 aiohttp/helpers.py    |  2 +-
 tests/test_helpers.py | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/aiohttp/helpers.py b/aiohttp/helpers.py
index 1a45a060ff1..fe5ea9b5ab6 100644
--- a/aiohttp/helpers.py
+++ b/aiohttp/helpers.py
@@ -332,7 +332,7 @@ def parse_mimetype(mimetype: str) -> MimeType:
     parts = mimetype.split(";")
     params: MultiDict[str] = MultiDict()
     for item in parts[1:]:
-        if not item:
+        if not item.strip():
             continue
         key, _, value = item.partition("=")
         params.add(key.lower().strip(), value.strip(' "'))
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index 4d6b9e34e1d..deaea0f9fa0 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -74,6 +74,27 @@
                 "text", "plain", "", MultiDictProxy(MultiDict({"base64": ""}))
             ),
         ),
+        (
+            "text/html; ",
+            helpers.MimeType("text", "html", "", MultiDictProxy(MultiDict())),
+        ),
+        (
+            "text/html;  ",
+            helpers.MimeType("text", "html", "", MultiDictProxy(MultiDict())),
+        ),
+        (
+            "text/html;\t",
+            helpers.MimeType("text", "html", "", MultiDictProxy(MultiDict())),
+        ),
+        (
+            "text/html; charset=utf-8; ",
+            helpers.MimeType(
+                "text",
+                "html",
+                "",
+                MultiDictProxy(MultiDict({"charset": "utf-8"})),
+            ),
+        ),
     ],
 )
 def test_parse_mimetype(mimetype: str, expected: helpers.MimeType) -> None: