diff --git a/src/google/adk/models/google_llm.py b/src/google/adk/models/google_llm.py
index 098388ef7e..60dee6f87f 100644
--- a/src/google/adk/models/google_llm.py
+++ b/src/google/adk/models/google_llm.py
@@ -58,6 +58,47 @@
 """
 
 
+_SUPPORTED_FILE_CONTENT_MIME_TYPES = frozenset({
+    # Images
+    'image/png',
+    'image/jpeg',
+    'image/webp',
+    'image/heic',
+    'image/heif',
+    # Documents & Text
+    'application/pdf',
+    'text/plain',
+    'text/csv',
+    'text/html',
+    'text/md',
+    'text/x-python',
+    'text/javascript',
+    # Audio
+    'audio/wav',
+    'audio/mp3',
+    'audio/aiff',
+    'audio/aac',
+    'audio/ogg',
+    'audio/flac',
+    'audio/mpeg',
+    'audio/mpga',
+    'audio/m4a',
+    'audio/pcm',
+    'audio/webm',
+    # Video
+    'video/mp4',
+    'video/mpeg',
+    'video/mov',
+    'video/quicktime',
+    'video/avi',
+    'video/x-flv',
+    'video/mpg',
+    'video/webm',
+    'video/wmv',
+    'video/3gpp',
+})
+
+
 class _ResourceExhaustedError(ClientError):
   """Represents a resources exhausted error received from the Model."""
 
@@ -455,9 +496,26 @@ async def _preprocess_request(self, llm_request: LlmRequest) -> None:
           for part in content.parts:
             # Create copies to avoid mutating the original objects
             if part.inline_data:
+              mime_type = (part.inline_data.mime_type or '').lower()
+              if mime_type not in _SUPPORTED_FILE_CONTENT_MIME_TYPES:
+                identifier = part.inline_data.display_name or 'inline_file'
+                part.text = (
+                    part.text or ''
+                ) + f'\n[File reference: "{identifier}"]'
+                part.inline_data = None
               part.inline_data = copy.copy(part.inline_data)
               _remove_display_name_if_present(part.inline_data)
+
             if part.file_data:
+              mime_type = (part.file_data.mime_type or '').lower()
+              identifier = (
+                  part.file_data.display_name or part.file_data.file_uri
+              )
+              if mime_type not in _SUPPORTED_FILE_CONTENT_MIME_TYPES:
+                part.text = (
+                    part.text or ''
+                ) + f'\n[File reference: "{identifier}"]'
+                part.file_data = None
               part.file_data = copy.copy(part.file_data)
               _remove_display_name_if_present(part.file_data)
 
diff --git a/tests/unittests/models/test_google_llm.py b/tests/unittests/models/test_google_llm.py
index d1ee05be1e..1ba35dc4e6 100644
--- a/tests/unittests/models/test_google_llm.py
+++ b/tests/unittests/models/test_google_llm.py
@@ -2167,3 +2167,49 @@ async def __aexit__(self, *args):
       # Verify the final speech_config is still None
       assert config_arg.speech_config is None
       assert isinstance(connection, GeminiLlmConnection)
+
+
+@pytest.mark.asyncio
+async def test_preprocess_request_unsupported_mime_type(gemini_llm):
+  """Verifies that MS Office files are escaped to a text reference."""
+  unsupported_part = types.Part(
+      file_data=types.FileData(
+          mime_type="application/vnd.ms-excel",
+          file_uri="gs://bucket/data.xls",
+          display_name="data.xls",
+      )
+  )
+  req = LlmRequest(
+      model="gemini-2.0-flash",
+      contents=[types.Content(parts=[unsupported_part])],
+  )
+
+  await gemini_llm._preprocess_request(req)
+
+  processed_part = req.contents[0].parts[0]
+  # File_data should be stripped to avoid the 400 error
+  assert processed_part.file_data is None
+  # Text fallback should be present
+  assert '[File reference: "data.xls"]' in processed_part.text
+
+
+@pytest.mark.asyncio
+async def test_preprocess_request_supported_mime_type(gemini_llm):
+  """Verifies that PDF files are passed through without modification."""
+  supported_part = types.Part(
+      file_data=types.FileData(
+          mime_type="application/pdf",
+          file_uri="gs://bucket/doc.pdf",
+          display_name="doc.pdf",
+      )
+  )
+  req = LlmRequest(
+      model="gemini-2.0-flash", contents=[types.Content(parts=[supported_part])]
+  )
+
+  await gemini_llm._preprocess_request(req)
+
+  processed_part = req.contents[0].parts[0]
+  # file_data should still be intact
+  assert processed_part.file_data is not None
+  assert processed_part.file_data.mime_type == "application/pdf"