How do you implement file upload with validation in Node.js?

Question

Accepted Answer

**Multer** — middleware for handling multipart/form-data (file uploads).

```javascript
const multer = require("multer");
const path = require("path");

// Storage configuration
const storage = multer.diskStorage({
    destination: (req, file, cb) => cb(null, "uploads/"),
    filename: (req, file, cb) => {
        const unique = Date.now() + "-" + Math.round(Math.random() * 1e9);
        cb(null, unique + path.extname(file.originalname));
    },
});

// File filter
const fileFilter = (req, file, cb) => {
    const allowed = /jpeg|jpg|png|gif/;
    const ext = allowed.test(path.extname(file.originalname).toLowerCase());
    const mime = allowed.test(file.mimetype);
    cb(null, ext && mime);
};

const upload = multer({
    storage,
    fileFilter,
    limits: { fileSize: 5 * 1024 * 1024 }, // 5MB
});

app.post("/upload", upload.single("image"), (req, res) => {
    res.json({ file: req.file });
});
```

**Security considerations:**
- Validate file type (don't trust MIME).
- Limit file size.
- Store outside public directory.
- Rename files to prevent path traversal.

Node.js: How do you implement file upload with validation in Node.js?