FastAPI File Upload

FastAPI is async to its core, so the right way to upload a file is with an async HTTP client — httpx.AsyncClient — not by wrapping a synchronous boto3 call in a thread pool. files.link is three awaited calls that fit cleanly into FastAPI's event loop: POST metadata, PUT bytes, POST confirm. No boto3, no aiobotocore, no AWS credentials chain, no bucket.

FastAPI gives you the incoming file as an UploadFile, a SpooledTemporaryFile-backed object with async read methods, so you can move data without blocking the loop:

import os, httpx
from fastapi import FastAPI, UploadFile

app = FastAPI()
KEY = os.environ['FILESLINK_KEY']
BASE = 'https://api.files.link'
FOLDER = os.environ['FILESLINK_FOLDER']

@app.post('/upload')
async def upload(file: UploadFile):
    data = await file.read()
    async with httpx.AsyncClient() as client:
        r1 = await client.post(f'{BASE}/v1/files/{FOLDER}',
            headers={'Authorization': KEY},
            json={'filesMetadata': [{'name': file.filename, 'size': len(data), 'type': file.content_type}]})
        slot = r1.json()['urls'][0]

        await client.put(slot['url'], content=data, headers={'Content-Type': file.content_type})

        await client.post(f'{BASE}/v1/files/confirm-upload',
            headers={'Authorization': KEY}, json={'ids': [slot['id']]})
    return {'id': slot['id']}

Reuse a single httpx.AsyncClient across requests (create it in a lifespan handler and store it on app.state) so connections are pooled instead of re-dialed on every upload. For uploads where you do not want the client waiting on the transfer, hand the three-call flow to BackgroundTasks so the response returns the moment metadata is accepted; for retry guarantees, push it to a task queue like arq or Celery instead.

Note that file.read() with no argument loads the whole file into memory — for large media, read in chunks with await file.read(chunk_size) and use the multipart endpoint, PUTting each part to its own presigned URL as you stream.

The Authorization header takes the raw API key (no Bearer prefix), sourced from pydantic-settings BaseSettings reading your .env and injected via Depends. files.link returns CDN-backed URLs with per-file visibility, storage encrypted at rest, and prepaid-credit billing — async upload without the boto3 footprint or a surprise egress bill.