diff --git a/src/kinto_http/client.py b/src/kinto_http/client.py
index 86f6a6d..35b7914 100644
--- a/src/kinto_http/client.py
+++ b/src/kinto_http/client.py
@@ -923,20 +923,20 @@ def add_attachment(
         permissions=None,
         mimetype=None,
     ):
-        with open(filepath, "rb") as f:
-            filecontent = f.read()
         filename = os.path.basename(filepath)
         if mimetype is None:
             mimetype, _ = mimetypes.guess_type(filepath)
-        multipart = [("attachment", (filename, filecontent, mimetype))]
         endpoint = self._get_endpoint("attachment", id=id, bucket=bucket, collection=collection)
-        resp, _ = self.session.request(
-            "post",
-            endpoint,
-            data=json.dumps(data) if data is not None else None,
-            permissions=json.dumps(permissions) if permissions is not None else None,
-            files=multipart,
-        )
+
+        with open(filepath, "rb") as file:
+            resp, _ = self.session.request(
+                "post",
+                endpoint,
+                data=json.dumps(data) if data is not None else None,
+                permissions=json.dumps(permissions) if permissions is not None else None,
+                files=[("attachment", (filename, file, mimetype))],
+            )
+
         return resp
 
     @retry_timeout
diff --git a/tests/test_client.py b/tests/test_client.py
index 3f52bf3..428bb20 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -3,6 +3,7 @@
 
 import pytest
 from pytest_mock.plugin import MockerFixture
+from unittest.mock import patch, mock_open
 
 from kinto_http import (
     BearerTokenAuth,
@@ -1427,22 +1428,21 @@ def test_add_attachment_guesses_mimetype(record_setup: Client, tmp_path):
     client = record_setup
     mock_response(client.session)
 
-    p = tmp_path / "file.txt"
-    p.write_text("hello")
-    client.add_attachment(
-        id="abc",
-        bucket="a",
-        collection="b",
-        filepath=p,
-    )
+    with patch("builtins.open", mock_open(read_data="hello")) as mock_file:
+        client.add_attachment(
+            id="abc",
+            bucket="a",
+            collection="b",
+            filepath="file.txt",
+        )
 
-    client.session.request.assert_called_with(
-        "post",
-        "/buckets/a/collections/b/records/abc/attachment",
-        data=None,
-        permissions=None,
-        files=[("attachment", ("file.txt", b"hello", "text/plain"))],
-    )
+        client.session.request.assert_called_with(
+            "post",
+            "/buckets/a/collections/b/records/abc/attachment",
+            data=None,
+            permissions=None,
+            files=[("attachment", ("file.txt", mock_file.return_value, "text/plain"))],
+        )
 
 
 def test_get_permissions(client_setup: Client):