diff --git a/contributing/samples/gepa/experiment.py b/contributing/samples/gepa/experiment.py
index f3751206a8..2710c3894c 100644
--- a/contributing/samples/gepa/experiment.py
+++ b/contributing/samples/gepa/experiment.py
@@ -43,7 +43,6 @@
 from tau_bench.types import EnvRunResult
 from tau_bench.types import RunConfig
 import tau_bench_agent as tau_bench_agent_lib
-
 import utils
 
 
diff --git a/contributing/samples/gepa/run_experiment.py b/contributing/samples/gepa/run_experiment.py
index d857da9635..e31db15788 100644
--- a/contributing/samples/gepa/run_experiment.py
+++ b/contributing/samples/gepa/run_experiment.py
@@ -25,7 +25,6 @@
 from absl import flags
 import experiment
 from google.genai import types
-
 import utils
 
 _OUTPUT_DIR = flags.DEFINE_string(
diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index dad5543fed..6a24ef3320 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -138,6 +138,28 @@
 # Providers that require file_id instead of inline file_data
 _FILE_ID_REQUIRED_PROVIDERS = frozenset({"openai", "azure"})
 
+# Default filenames for file uploads when display_name is missing.
+# OpenAI derives the MIME type from the filename extension during upload,
+# so passing a proper name ensures the stored file gets the right content-type.
+_MIME_TO_FILENAME = {
+    "application/pdf": "document.pdf",
+    "application/json": "document.json",
+    "application/msword": "document.doc",
+    "application/vnd.openxmlformats-officedocument.wordprocessingml.document": (
+        "document.docx"
+    ),
+    "application/vnd.openxmlformats-officedocument.presentationml.presentation": (
+        "document.pptx"
+    ),
+    "application/x-sh": "script.sh",
+}
+
+
+def _filename_for_mime(mime_type: str) -> str:
+  """Return a default filename for a MIME type so uploads get the right content-type."""
+  return _MIME_TO_FILENAME.get(mime_type, "document.bin")
+
+
 _MISSING_TOOL_RESULT_MESSAGE = (
     "Error: Missing tool result (tool execution may have been interrupted "
     "before a response was recorded)."
@@ -840,10 +862,18 @@ async def _get_content(
             url_content_type: {"url": data_uri},
         })
       elif mime_type in _SUPPORTED_FILE_CONTENT_MIME_TYPES:
-        # OpenAI/Azure require file_id from uploaded file, not inline data
+        # OpenAI/Azure require file_id from uploaded file, not inline data.
+        # Pass (filename, content, content_type) so the upload gets the right MIME type.
         if provider in _FILE_ID_REQUIRED_PROVIDERS:
+          display_name = getattr(
+              part.inline_data, "display_name", None
+          ) or _filename_for_mime(part.inline_data.mime_type)
           file_response = await litellm.acreate_file(
-              file=part.inline_data.data,
+              file=(
+                  display_name,
+                  part.inline_data.data,
+                  part.inline_data.mime_type,
+              ),
               purpose="assistants",
               custom_llm_provider=provider,
           )
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 8e353efb21..09cf7b0c04 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -3958,7 +3958,29 @@ async def test_get_content_pdf_openai_uses_file_id(mocker):
   assert "file_data" not in content[0]["file"]
 
   mock_acreate_file.assert_called_once_with(
-      file=b"test_pdf_data",
+      file=("document.pdf", b"test_pdf_data", "application/pdf"),
+      purpose="assistants",
+      custom_llm_provider="openai",
+  )
+
+
+@pytest.mark.asyncio
+async def test_get_content_pdf_openai_uses_display_name_as_filename(mocker):
+  """Test that display_name is used as filename when available."""
+  mock_file_response = mocker.create_autospec(litellm.FileObject)
+  mock_file_response.id = "file-abc123"
+  mock_acreate_file = AsyncMock(return_value=mock_file_response)
+  mocker.patch.object(litellm, "acreate_file", new=mock_acreate_file)
+
+  part = types.Part.from_bytes(
+      data=b"test_pdf_data", mime_type="application/pdf"
+  )
+  part.inline_data.display_name = "my_report.pdf"
+  content = await _get_content([part], provider="openai")
+
+  assert content[0]["file"]["file_id"] == "file-abc123"
+  mock_acreate_file.assert_called_once_with(
+      file=("my_report.pdf", b"test_pdf_data", "application/pdf"),
       purpose="assistants",
       custom_llm_provider="openai",
   )
@@ -3997,7 +4019,7 @@ async def test_get_content_pdf_azure_uses_file_id(mocker):
   assert content[0]["file"]["file_id"] == "file-xyz789"
 
   mock_acreate_file.assert_called_once_with(
-      file=b"test_pdf_data",
+      file=("document.pdf", b"test_pdf_data", "application/pdf"),
       purpose="assistants",
       custom_llm_provider="azure",
   )
@@ -4041,7 +4063,11 @@ async def test_get_completion_inputs_openai_file_upload(mocker):
   assert content[1]["type"] == "file"
   assert content[1]["file"]["file_id"] == "file-uploaded123"
 
-  mock_acreate_file.assert_called_once()
+  mock_acreate_file.assert_called_once_with(
+      file=("document.pdf", b"test_pdf_content", "application/pdf"),
+      purpose="assistants",
+      custom_llm_provider="openai",
+  )
 
 
 @pytest.mark.asyncio