From f67e98fd789fa8b4038250d28e32d1a51495ba92 Mon Sep 17 00:00:00 2001
From: JSap0914 <116227558+JSap0914@users.noreply.github.com>
Date: Wed, 8 Jul 2026 09:13:06 +0900
Subject: [PATCH] Fix parse_mimetype producing spurious empty-key parameter for
 whitespace-only segments after semicolons (#13010)

(cherry picked from commit 9e00085c958fbcd40de37867e773bf7cd3bbee0c)
---
 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 486b4976cac..fbdd6653fd0 100644
--- a/aiohttp/helpers.py
+++ b/aiohttp/helpers.py
@@ -373,7 +373,7 @@ 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 9c1193a7a87..197cbfaf61c 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -60,6 +60,27 @@
             "text/plain;base64",
             helpers.MimeType("text", "plain", "", 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, expected) -> None: