[PR #10270/2a55afee backport][3.12] fix: include carriage return in multipartform.decode() (#10275)

**This is a backport of PR #10270 as merged into master
(2a55afe).**

Co-authored-by: James Ward <[email protected]>

Author: patchback[bot]

CHANGES/10270.bugfix.rst

@@ -0,0 +1,2 @@
+``MultipartForm.decode()`` must follow RFC1341 7.2.1 with a ``CRLF`` after the boundary
+-- by :user:`imnotjames`.

CONTRIBUTORS.txt

@@ -168,6 +168,7 @@ Jaesung Lee
 Jake Davis
 Jakob Ackermann
 Jakub Wilk
+James Ward
 Jan Buchar
 Jan Gosmann
 Jarno Elonen

aiohttp/multipart.py

@@ -979,7 +979,7 @@ def decode(self, encoding: str = "utf-8", errors: str = "strict") -> str:
         return "".join(
             "--"
             + self.boundary
-            + "\n"
+            + "\r\n"
             + part._binary_headers.decode(encoding, errors)
             + part.decode()
             for part, _e, _te in self._parts

tests/test_web_response.py

@@ -1209,7 +1209,7 @@ def read(self, size: int = -1) -> bytes:
         (BodyPartReader("x", CIMultiDictProxy(CIMultiDict()), mock.Mock()), None),
         (
             mpwriter,
-            "--x\nContent-Type: text/plain; charset=utf-8\r\nContent-Length: 4\r\n\r\ntest",
+            "--x\r\nContent-Type: text/plain; charset=utf-8\r\nContent-Length: 4\r\n\r\ntest",
         ),
     ),
 )