diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c1ecb0..c6b23c3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- `append_datasource` with a local `file` now names the multipart form field after the detected format (`csv`, `ndjson` or `parquet`) instead of always using `csv`, so NDJSON and Parquet uploads are accepted by the API.
+
 ## [0.4.0] - 2026-06-29
 
 ### Added
diff --git a/src/tinybird_sdk/api/api.py b/src/tinybird_sdk/api/api.py
index 8ee18b5..2c4cdf5 100644
--- a/src/tinybird_sdk/api/api.py
+++ b/src/tinybird_sdk/api/api.py
@@ -272,7 +272,7 @@ def append_datasource(
             with open(file_path_str, "rb") as fp:
                 file_content = fp.read()
             content_type, multipart = create_multipart_body(
-                files=[("csv", file_path_str, file_content, None)],
+                files=[(detected_format or "csv", file_path_str, file_content, None)],
             )
             response = self.request(
                 f"/v0/datasources?{urlencode(query)}",
diff --git a/tests/test_api_parity.py b/tests/test_api_parity.py
index 6f13da0..095092f 100644
--- a/tests/test_api_parity.py
+++ b/tests/test_api_parity.py
@@ -124,6 +124,35 @@ def fake_fetch(url: str, **kwargs: Any) -> _FakeResponse:
     assert api.truncate_datasource("events") == {}
 
 
+@pytest.mark.parametrize(
+    ("filename", "field_name"),
+    [
+        ("events.csv", "csv"),
+        ("events.ndjson", "ndjson"),
+        ("events.jsonl", "ndjson"),
+        ("events.parquet", "parquet"),
+        ("events.dat", "csv"),
+    ],
+)
+def test_append_file_uses_format_as_multipart_field_name(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Path, filename: str, field_name: str
+) -> None:
+    calls: list[dict[str, Any]] = []
+
+    def fake_fetch(_url: str, **kwargs: Any) -> _FakeResponse:
+        calls.append(kwargs)
+        return _FakeResponse(200, {"ok": True})
+
+    monkeypatch.setattr(api_module, "tinybird_fetch", fake_fetch)
+    api = TinybirdApi({"base_url": "https://api.tinybird.co", "token": "p.token"})
+
+    local_file = tmp_path / filename
+    local_file.write_bytes(b"data")
+    api.append_datasource("events", {"file": str(local_file)})
+
+    assert f'name="{field_name}"; filename="{filename}"'.encode() in calls[0]["body"]
+
+
 def test_append_requires_either_url_or_file() -> None:
     api = TinybirdApi({"base_url": "https://api.tinybird.co", "token": "p.token"})
     with pytest.raises(ValueError, match="Either 'url' or 'file'"):