Fix prepare_body stream detection for __getattr__-based file wrappers (#7433)

---------

Co-authored-by: Nate Prewitt <nate.prewitt@gmail.com>

Author: k223kim

src/requests/models.py

@@ -596,9 +596,9 @@ def prepare_body(
             if not isinstance(body, bytes):
                 body = body.encode("utf-8")
 
-        if isinstance(data, Iterable) and not isinstance(
-            data, (str, bytes, list, tuple, Mapping)
-        ):
+        # data that proxies attributes to underlying objects needs hasattr
+        is_iterable = isinstance(data, Iterable) or hasattr(data, "__iter__")
+        if is_iterable and not isinstance(data, (str, bytes, list, tuple, Mapping)):
             try:
                 length = super_len(data)
             except (TypeError, AttributeError, UnsupportedOperation):

tests/test_requests.py

@@ -2073,6 +2073,21 @@ def __iter__(self):
 
         assert "Unable to rewind request body" in str(e)
 
+    def test_getattr_proxy_stream_follows_redirect(self, httpbin):
+        """Ensure stream wrappers that don't implement __iter__ directly are still detected."""
+
+        class AttrProxy:
+            def __init__(self):
+                self._file = io.BytesIO(b"data")
+
+            def __getattr__(self, name):
+                return getattr(self._file, name)
+
+        r = requests.post(
+            httpbin("redirect-to?url=/post&status_code=307"), data=AttrProxy()
+        )
+        assert r.json()["data"] == "data"
+
     def _patch_adapter_gzipped_redirect(self, session, url):
         adapter = session.get_adapter(url=url)
         org_build_response = adapter.build_response