fix(chat): increment temp file suffix correctly

core/chat/message_elements.py

@@ -70,13 +70,13 @@ def _move_no_clobber(source: str, candidate: str) -> Optional[str]:
     numbered candidate. Returns the final path, or None when the move was not
     possible (e.g. filesystems without hard link support).
     """
+    root, ext = os.path.splitext(candidate)
     counter = 1
     while True:
         try:
             os.link(source, candidate)
             break
         except FileExistsError:
-            root, ext = os.path.splitext(candidate)
             candidate = f"{root}_{counter}{ext}"
             counter += 1
         except OSError:

tests/test_message_elements.py

@@ -7,6 +7,7 @@
 from core.chat.message_elements import (
     _build_temp_file_path,
     _infer_mime_from_bytes,
+    _move_no_clobber,
     check_base64,
     ElementType,
     Text,
@@ -25,6 +26,24 @@
 )
 
 
+# ── _move_no_clobber ─────────────────────────────────────────────
+
+def test_move_no_clobber_increments_from_original_candidate(tmp_path):
+    source = tmp_path / "source"
+    source.write_bytes(b"new")
+    candidate = tmp_path / "download.png"
+    candidate.write_bytes(b"original")
+    (tmp_path / "download_1.png").write_bytes(b"first duplicate")
+
+    result = _move_no_clobber(str(source), str(candidate))
+
+    assert result == str(tmp_path / "download_2.png")
+    assert not source.exists()
+    assert candidate.read_bytes() == b"original"
+    assert (tmp_path / "download_1.png").read_bytes() == b"first duplicate"
+    assert (tmp_path / "download_2.png").read_bytes() == b"new"
+
+
 # ── _infer_mime_from_bytes ──────────────────────────────────────────
 
 def test_infer_png():