diff --git a/python/lib/sift_client/_internal/low_level_wrappers/upload.py b/python/lib/sift_client/_internal/low_level_wrappers/upload.py
index e1c9c6777..4b9e420ef 100644
--- a/python/lib/sift_client/_internal/low_level_wrappers/upload.py
+++ b/python/lib/sift_client/_internal/low_level_wrappers/upload.py
@@ -93,7 +93,11 @@ async def upload_attachment(
         file_name, mimetype, content_encoding = self._mime_and_content_type_from_path(posix_path)
 
         if not mimetype:
-            raise ValueError(f"The MIME-type of '{posix_path}' could not be computed.")
+            extension = posix_path.suffix
+            if extension:
+                mimetype = f"application/x-{extension.lstrip('.')}"
+            else:
+                mimetype = "application/octet-stream"  # fallback to generic 'binary data' MIME type
 
         # Run the synchronous file upload in a thread pool to avoid blocking the event loop
         loop = asyncio.get_event_loop()
diff --git a/python/lib/sift_client/_tests/_internal/low_level_wrappers/test_upload.py b/python/lib/sift_client/_tests/_internal/low_level_wrappers/test_upload.py
index d40fd9aec..c9b511174 100644
--- a/python/lib/sift_client/_tests/_internal/low_level_wrappers/test_upload.py
+++ b/python/lib/sift_client/_tests/_internal/low_level_wrappers/test_upload.py
@@ -1,16 +1,62 @@
 """Tests for UploadLowLevelClient functionality."""
 
 from pathlib import Path
+from unittest.mock import patch
+
+import pytest
 
 from sift_client._internal.low_level_wrappers.upload import UploadLowLevelClient
 
 
-class TestUploadLowLevelClient:
-    class TestMimeAndContentTypeFromPath:
-        def test_parquet_file_extension(self):
-            _, mime, _ = UploadLowLevelClient._mime_and_content_type_from_path(Path("data.parquet"))
-            assert mime == "application/vnd.apache.parquet"
+class TestUploadAttachment:
+    @pytest.mark.asyncio
+    async def test_known_mime_type(self, tmp_path):
+        test_file = tmp_path / "video.mp4"
+        test_file.write_bytes(b"fake data")
+
+        client = UploadLowLevelClient.__new__(UploadLowLevelClient)
+
+        with patch.object(client, "_upload_file_sync", return_value="remote-file-123") as mock:
+            await client.upload_attachment(path=test_file, entity_id="e1", entity_type="runs")
+
+        _, _, mimetype, *_ = mock.call_args[0]
+        assert mimetype == "video/mp4"
+
+    @pytest.mark.asyncio
+    async def test_unknown_extension_falls_back_to_application_x_ext(self, tmp_path):
+        test_file = tmp_path / "data.pcapng"
+        test_file.write_bytes(b"fake data")
+
+        client = UploadLowLevelClient.__new__(UploadLowLevelClient)
+
+        with patch.object(client, "_upload_file_sync", return_value="remote-file-123") as mock:
+            await client.upload_attachment(path=test_file, entity_id="e1", entity_type="runs")
+
+        _, _, mimetype, *_ = mock.call_args[0]
+        assert mimetype == "application/x-pcapng"
+
+    @pytest.mark.asyncio
+    async def test_no_extension_falls_back_to_octet_stream(self, tmp_path):
+        test_file = tmp_path / "README"
+        test_file.write_bytes(b"fake data")
+
+        client = UploadLowLevelClient.__new__(UploadLowLevelClient)
+
+        with patch.object(client, "_upload_file_sync", return_value="remote-file-123") as mock:
+            await client.upload_attachment(path=test_file, entity_id="e1", entity_type="runs")
+
+        _, _, mimetype, *_ = mock.call_args[0]
+        assert mimetype == "application/octet-stream"
+
+    @pytest.mark.asyncio
+    async def test_file_name_preserved(self, tmp_path):
+        test_file = tmp_path / "my_file.pcapng"
+        test_file.write_bytes(b"fake data")
+
+        client = UploadLowLevelClient.__new__(UploadLowLevelClient)
+
+        with patch.object(client, "_upload_file_sync", return_value="remote-file-123") as mock:
+            await client.upload_attachment(path=test_file, entity_id="e1", entity_type="runs")
 
-        def test_pqt_file_extension(self):
-            _, mime, _ = UploadLowLevelClient._mime_and_content_type_from_path(Path("data.pqt"))
-            assert mime == "application/vnd.apache.parquet"
+        file_name, *_ = mock.call_args[0]
+        assert file_name == Path(test_file)