## Description

When uploading a file via `POST /v1/files` using the `x-litellm-model` header, LiteLLM returns an encoded file ID. However, when this encoded file ID is passed in `/chat/completions` or `/v1/responses`, LiteLLM does not decode it before forwarding to the provider — causing the requests to fail.

Note: this issue is specific to the `x-litellm-model` header upload path. Uploading via the `target_model_names` form field (managed files path) returns an encoded ID that **does** work transparently in chat completions.

## Steps to Reproduce

**1. Upload a file using x-litellm-model header:** ```bash curl -X POST http://localhost:4000/v1/files \ -H "Authorization: Bearer sk-1234" \ -H "x-litellm-model: gpt-5.1" \ -F purpose="assistants" \ -F file="@receipt.pdf" ```

Response: ```json { "id": "file-bGl0ZWxsbTpmaWxlLUV4VHVDYXdVcXhFTWpWRks2eHdSOUI7bW9kZWwsZ3B0LTUuMQ", ... } ```

**2. Use the returned file ID in chat completions:** ```bash curl -X POST http://localhost:4000/v1/chat/completions \ -H "Authorization: Bearer sk-1234" \ -H "Content-Type: application/json" \ -d '{ "model": "gpt-5.1", "messages": [{"role": "user", "content": [ {"type":…