File Uploads in FastAPI — Simplified for Developers

Uploading files shouldn’t feel like decoding rocket science.

File Uploads in FastAPI — Simplified for Developers

Uploading files is one of those features every web app needs but no one really enjoys implementing — especially when every framework has its own quirks. But if you’re working with FastAPI, you’re in luck.

FastAPI makes file uploads beautifully simple, yet powerful — once you know the essentials. In this guide, I’ll break down how to handle file uploads in FastAPI the right way, with minimal boilerplate and maximum clarity.

Setting Up the Basics

First things first, install FastAPI and Uvicorn:

pip install fastapi uvicorn python-multipart

Yes, python-multipart is required for parsing multipart/form-data used in file uploads.

Here’s the simplest example to get a file from a client:

from fastapi import FastAPI, File, UploadFile 
 
app = FastAPI() 
 
@app.post("/upload/") 
async def upload_file(file: UploadFile = File(...)): 
    contents = await file.read() 
    return {"filename": file.filename, "content_type": file.content_type}

• UploadFile gives you access to the file's metadata and contents without loading the entire file into memory.
• File(...) tells FastAPI this is a required file field.
• await file.read() is how you get the raw content.

Upload vs UploadFile: Which One to Use?

FastAPI gives you two options:

| Type         | Use Case     | Memory Usage                            | 
| ------------ | ------------ | --------------------------------------- | 
| bytes        | Small files  | Loaded into memory                      | 
| UploadFile   | Larger files | Uses temporary file, better performance |

Prefer UploadFile for most cases — it's more scalable and memory-efficient.

Saving the File to Disk

Need to store the uploaded file somewhere? Here’s how:

import shutil 
 
@app.post("/save/") 
async def save_file(file: UploadFile = File(...)): 
    with open(f"uploads/{file.filename}", "wb") as buffer: 
        shutil.copyfileobj(file.file, buffer) 
    return {"message": f"{file.filename} saved successfully"}

Make sure the uploads/ folder exists, or add logic to create it if missing.

Upload Multiple Files

FastAPI makes handling multiple uploads equally simple:

from typing import List 
 
@app.post("/multi-upload/") 
async def multi_upload(files: List[UploadFile] = File(...)): 
    return {"filenames": [file.filename for file in files]}

Now your endpoint supports drag-and-drop uploads or multi-file form fields with no extra complexity.

File Validation (Size, Type, etc.)

Want to accept only PDFs under 2MB? Here’s a way to do it:

@app.post("/secure-upload/") 
async def secure_upload(file: UploadFile = File(...)): 
    if not file.filename.endswith(".pdf"): 
        return {"error": "Only PDF files allowed"} 
     
    contents = await file.read() 
    if len(contents) > 2 * 1024 * 1024: 
        return {"error": "File too large"} 
 
    return {"message": "File accepted"}

For advanced validation, consider integrating with Pydantic models, custom validators, or even virus scanners like ClamAV.

Testing File Uploads

Use Swagger UI (auto-generated at /docs) to test uploads manually. It even provides a file picker!

Or use curl:

curl -F "file=@example.pdf" http://localhost:8000/upload/

Or from JavaScript:

const formData = new FormData(); 
formData.append("file", fileInput.files[0]); 
 
fetch("/upload", { 
  method: "POST", 
  body: formData, 
});

Wrapping Up

FastAPI makes file uploads uncomplicated, secure, and developer-friendly — especially compared to legacy frameworks. With just a few lines of code, you can handle everything from single file uploads to multi-file validations and storage.

Whether you’re building an image hosting API, a resume upload feature, or a bulk CSV import tool — FastAPI has your back.

What’s Next?
If you found this helpful, consider bookmarking it and sharing it with your dev team. And if you’re curious about file streaming, background processing, or S3 integrations in FastAPI — let me know, and I’ll cover that next!