Content block corruption when using type file

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [x] langchain - [x] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python from traceback import format_exc from langchain_core.callbacks.base import AsyncCallbackHandler from langchain_core.messages import HumanMessage from langchain_core.messages.content import create_file_block from langchain_core.runnables import RunnableConfig from langchain_openai import ChatOpenAI class CustomHandler(AsyncCallbackHandler): async def on_chat_model_start( self, serialized, messages, *, run_id, parent_run_id=None, tags=None, metadata=None, invocation_params: dict | None = None, **kwargs, ): for i, message in enumerate(messages[0]): print(f'Message {i}') try: message.content_blocks except Excepti…