diff --git a/api/core/tools/utils/message_transformer.py b/api/core/tools/utils/message_transformer.py
index ca2aa39861..df322eda1c 100644
--- a/api/core/tools/utils/message_transformer.py
+++ b/api/core/tools/utils/message_transformer.py
@@ -101,6 +101,8 @@ class ToolFileMessageTransformer:
                 meta = message.meta or {}
 
                 mimetype = meta.get("mime_type", "application/octet-stream")
+                if not mimetype:
+                    mimetype = "application/octet-stream"
                 # get filename from meta
                 filename = meta.get("filename", None)
                 # if message is str, encode it to bytes
diff --git a/api/tests/unit_tests/core/tools/utils/test_message_transformer.py b/api/tests/unit_tests/core/tools/utils/test_message_transformer.py
new file mode 100644
index 0000000000..af3cdddd5f
--- /dev/null
+++ b/api/tests/unit_tests/core/tools/utils/test_message_transformer.py
@@ -0,0 +1,86 @@
+import pytest
+
+import core.tools.utils.message_transformer as mt
+from core.tools.entities.tool_entities import ToolInvokeMessage
+
+
+class _FakeToolFile:
+    def __init__(self, mimetype: str):
+        self.id = "fake-tool-file-id"
+        self.mimetype = mimetype
+
+
+class _FakeToolFileManager:
+    """Fake ToolFileManager to capture the mimetype passed in."""
+
+    last_call: dict | None = None
+
+    def __init__(self, *args, **kwargs):
+        pass
+
+    def create_file_by_raw(
+        self,
+        *,
+        user_id: str,
+        tenant_id: str,
+        conversation_id: str | None,
+        file_binary: bytes,
+        mimetype: str,
+        filename: str | None = None,
+    ):
+        type(self).last_call = {
+            "user_id": user_id,
+            "tenant_id": tenant_id,
+            "conversation_id": conversation_id,
+            "file_binary": file_binary,
+            "mimetype": mimetype,
+            "filename": filename,
+        }
+        return _FakeToolFile(mimetype)
+
+
+@pytest.fixture(autouse=True)
+def _patch_tool_file_manager(monkeypatch):
+    # Patch the manager used inside the transformer module
+    monkeypatch.setattr(mt, "ToolFileManager", _FakeToolFileManager)
+    # also ensure predictable URL generation (no need to patch; uses id and extension only)
+    yield
+    _FakeToolFileManager.last_call = None
+
+
+def _gen(messages):
+    yield from messages
+
+
+def test_transform_tool_invoke_messages_mimetype_key_present_but_none():
+    # Arrange: a BLOB message whose meta contains a mime_type key set to None
+    blob = b"hello"
+    msg = ToolInvokeMessage(
+        type=ToolInvokeMessage.MessageType.BLOB,
+        message=ToolInvokeMessage.BlobMessage(blob=blob),
+        meta={"mime_type": None, "filename": "greeting"},
+    )
+
+    # Act
+    out = list(
+        mt.ToolFileMessageTransformer.transform_tool_invoke_messages(
+            messages=_gen([msg]),
+            user_id="u1",
+            tenant_id="t1",
+            conversation_id="c1",
+        )
+    )
+
+    # Assert: default to application/octet-stream when mime_type is present but None
+    assert _FakeToolFileManager.last_call is not None
+    assert _FakeToolFileManager.last_call["mimetype"] == "application/octet-stream"
+
+    # Should yield a BINARY_LINK (not IMAGE_LINK) and the URL ends with .bin
+    assert len(out) == 1
+    o = out[0]
+    assert o.type == ToolInvokeMessage.MessageType.BINARY_LINK
+    assert isinstance(o.message, ToolInvokeMessage.TextMessage)
+    assert o.message.text.endswith(".bin")
+    # meta is preserved (still contains mime_type: None)
+    assert "mime_type" in (o.meta or {})
+    assert o.meta["mime_type"] is None