Upload a file

You're Claude Code, working in my repository.

Task
Add an upload_file helper to my app that POSTs a local file to https://api.meta.ai/v1/files as multipart with purpose user_data (and an optional expires_after), returns the FileObject, and raises on HTTP errors.

Read this first
Meta's docs page "Upload a file" as Markdown: https://dev.meta.ai/docs/api-reference/files/upload-file.md
It's the source of truth for request and response shapes. The same page for people: https://dev.meta.ai/docs/api-reference/files/upload-file

Setup
- Base URL: https://api.meta.ai/v1
- Auth: send Authorization: Bearer $MODEL_API_KEY, reading MODEL_API_KEY from the environment. Never hard-code, print or commit the key.
- Model: muse-spark-1.3-contributor (Contributor mode: Meta may use what you send to train future models; drop -contributor for Standard mode)
- Python: the OpenAI SDK (pip install openai) with base_url="https://api.meta.ai/v1" and api_key=os.environ["MODEL_API_KEY"] works, or plain requests.

Facts to get right
- Multipart fields: file and purpose (both required): user_data for files requests reference, batch for Batch input; other purposes (fine-tune) are 400s.
- expires_after is optional: expires_after[anchor]=created_at and expires_after[seconds] from 3600 to 2592000, both or neither. Without it the file never expires.
- Send purpose and expires_after before the file part (file last) for lower latency.
- The reply is a FileObject: {"id": "file-<digits>", "bytes", "created_at", "expires_at", "filename", "object": "file", "purpose", "status"}; expires_at may be null or missing without expires_after.
- Up to 1 GiB per file; uploads count toward the team's 100 GiB until deleted.

Starter code (Python)
import os

import requests


def upload_file(path, expires_seconds=None):
    data = {"purpose": "user_data"}
    if expires_seconds:
        data |= {"expires_after[anchor]": "created_at", "expires_after[seconds]": str(expires_seconds)}
    with open(path, "rb") as f:
        response = requests.post(
            "https://api.meta.ai/v1/files",
            headers={"Authorization": f"Bearer {os.environ['MODEL_API_KEY']}"},
            data=data,
            files={"file": f},
        )
    response.raise_for_status()
    return response.json()


print(upload_file("/path/to/image.png", expires_seconds=86400))

Done when
- upload_file('photo.png') returns an id that starts with file-.
- With expires_after seconds 86400, expires_at equals created_at + 86400.
- The key comes from MODEL_API_KEY and never appears in code, logs or commits.
- API errors (4xx, 429) surface their message instead of failing silently.