From 5a6949da642d1db6cf414fd0d1f70e54c7b7be14 Mon Sep 17 00:00:00 2001
From: Sam Bull <git@sambull.org>
Date: Mon, 15 Apr 2024 21:16:28 +0100
Subject: [PATCH] Add Content-Disposition automatically (#8335)

---
 CHANGES/8335.bugfix.rst |  1 +
 aiohttp/multipart.py    |  4 ++++
 tests/test_multipart.py | 22 +++++++++++++++++-----
 3 files changed, 22 insertions(+), 5 deletions(-)
 create mode 100644 CHANGES/8335.bugfix.rst

diff --git a/CHANGES/8335.bugfix.rst b/CHANGES/8335.bugfix.rst
new file mode 100644
index 00000000000..cd93b864a50
--- /dev/null
+++ b/CHANGES/8335.bugfix.rst
@@ -0,0 +1 @@
+Added default Content-Disposition in multipart/form-data responses -- by :user:`Dreamsorcerer`.
diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index 6a441ba6456..73a6caa33aa 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -879,6 +879,10 @@ def append_payload(self, payload: Payload) -> Payload:
                 not {CONTENT_ENCODING, CONTENT_LENGTH, CONTENT_TRANSFER_ENCODING}
                 & payload.headers.keys()
             )
+            # Set default Content-Disposition in case user doesn't create one
+            if CONTENT_DISPOSITION not in payload.headers:
+                name = f"section-{len(self._parts)}"
+                payload.set_content_disposition("form-data", name=name)
         else:
             # compression
             encoding = payload.headers.get(CONTENT_ENCODING, "").lower()
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index 79ee464c0e0..15bb0c320c6 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -1165,12 +1165,24 @@ def test_append_multipart(self, writer: Any) -> None:
         part = writer._parts[0][0]
         assert part.headers[CONTENT_TYPE] == "test/passed"
 
-    async def test_set_content_disposition_after_append(self):
+    def test_set_content_disposition_after_append(self):
         writer = aiohttp.MultipartWriter("form-data")
-        payload = writer.append("some-data")
-        payload.set_content_disposition("form-data", name="method")
-        assert CONTENT_DISPOSITION in payload.headers
-        assert "name=" in payload.headers[CONTENT_DISPOSITION]
+        part = writer.append("some-data")
+        part.set_content_disposition("form-data", name="method")
+        assert 'name="method"' in part.headers[CONTENT_DISPOSITION]
+
+    def test_automatic_content_disposition(self):
+        writer = aiohttp.MultipartWriter("form-data")
+        writer.append_json(())
+        part = payload.StringPayload("foo")
+        part.set_content_disposition("form-data", name="second")
+        writer.append_payload(part)
+        writer.append("foo")
+
+        disps = tuple(p[0].headers[CONTENT_DISPOSITION] for p in writer._parts)
+        assert 'name="section-0"' in disps[0]
+        assert 'name="second"' in disps[1]
+        assert 'name="section-2"' in disps[2]
 
     def test_with(self) -> None:
         with aiohttp.MultipartWriter(boundary=":") as writer: