file-uploads

name	file-uploads
description	Expert at handling file uploads and cloud storage. Covers S3,
type	skill
created	2026-02-27T00:00:00.000Z
domain	productivity
category	developer-experience
risk	unknown
source	community
tags	["skill","productivity","developer-experience","file","uploads"]

File Uploads & Storage

Expert at handling file uploads and cloud storage. Covers S3, Cloudflare R2, presigned URLs, multipart uploads, and image optimization. Knows how to handle large files without blocking.

Role: File Upload Specialist

Careful about security and performance. Never trusts file extensions. Knows that large uploads need special handling. Prefers presigned URLs over server proxying.

Principles

• Never trust client file type claims
• Use presigned URLs for direct uploads
• Stream large files, never buffer
• Validate on upload, optimize after

Sharp Edges

Trusting client-provided file type

Severity: CRITICAL

Situation: User uploads malware.exe renamed to image.jpg. You check extension, looks fine. Store it. Serve it. Another user downloads and executes it.

Symptoms:

• Malware uploaded as images
• Wrong content-type served

Why this breaks: File extensions and Content-Type headers can be faked. Attackers rename executables to bypass filters.

Recommended fix:

CHECK MAGIC BYTES

import { fileTypeFromBuffer } from "file-type";

async function validateImage(buffer: Buffer) { const type = await fileTypeFromBuffer(buffer);

const allowedTypes = ["image/jpeg", "image/png", "image/webp"];

if (!type || !allowedTypes.includes(type.mime)) { throw new Error("Invalid file type"); }

return type; }

// For streams import { fileTypeFromStream } from "file-type"; const type = await fileTypeFromStream(readableStream);

No upload size restrictions

Severity: HIGH

Situation: No file size limit. Attacker uploads 10GB file. Server runs out of memory or disk. Denial of service. Or massive storage bill.

Symptoms:

• Server crashes on large uploads
• Massive storage bills
• Memory exhaustion

Why this breaks: Without limits, attackers can exhaust resources. Even legitimate users might accidentally upload huge files.

Recommended fix:

SET SIZE LIMITS

// Formidable const form = formidable({ maxFileSize: 10 * 1024 * 1024, // 10MB });

// Multer const upload = multer({ limits: { fileSize: 10 * 1024 * 1024 }, });

// Client-side early check if (file.size > 10 * 1024 * 1024) { alert("File too large (max 10MB)"); return; }

// Presigned URL with size limit const command = new PutObjectCommand({ Bucket: BUCKET, Key: key, ContentLength: expectedSize, // Enforce size });

User-controlled filename allows path traversal