diff --git a/src/labapi/entry/entries/attachment.py b/src/labapi/entry/entries/attachment.py
index 05c40b1..df1425f 100644
--- a/src/labapi/entry/entries/attachment.py
+++ b/src/labapi/entry/entries/attachment.py
@@ -6,6 +6,7 @@
 
 from __future__ import annotations
 
+import shutil
 from email.message import Message
 from io import BytesIO
 from tempfile import TemporaryFile
@@ -89,7 +90,7 @@ def get_attachment(self, use_tempfile: bool = False) -> Attachment:
         # Return an independent copy so each caller gets isolated read/seek/close state
         # while still sharing a single downloaded backing attachment in the cache.
         self._filedata.seek(0)
-        output.write(self._filedata.read())
+        shutil.copyfileobj(self._filedata, output)
         output.seek(0)
 
         return Attachment(
diff --git a/tests/entry/entries/test_attachment.py b/tests/entry/entries/test_attachment.py
index 51b0da0..f89f292 100644
--- a/tests/entry/entries/test_attachment.py
+++ b/tests/entry/entries/test_attachment.py
@@ -169,3 +169,23 @@ def test_attachment_entry_get_attachment_caching(self, client, user: User):
         attachment3 = entry.get_attachment()
         assert client.stream_api_get.call_count == 1
         assert attachment3.read() == b"Content"
+
+    def test_get_attachment_tempfile_copies_in_chunks(self, client, user: User):
+        """get_attachment(use_tempfile=True) must not read the full payload at once.
+
+        Regression for #215: the old code called self._filedata.read() with no
+        size, materializing the entire cached payload as one bytes object.
+        """
+        entry = AttachmentEntry("eid_att", "Caption", user)
+
+        mock_response = Mock()
+        mock_response.headers = {
+            "Content-Type": "text/plain",
+            "Content-Disposition": 'attachment; filename="test.txt"',
+        }
+        mock_response.iter_content.return_value = [b"Chunked content"]
+        client.stream_api_get = Mock(return_value=StreamingResponse(mock_response))
+
+        attachment = entry.get_attachment(use_tempfile=True)
+        assert attachment.read() == b"Chunked content"
+        attachment.close()