File Uploads in Express

File uploads need careful handling.

An upload endpoint should not blindly accept any file of any size.

What upload code must decide

• maximum file size,
• allowed file types,
• storage destination,
• file naming,
• auth requirements,
• error handling,
• cleanup on failure.

Common middleware approach

Express apps often use upload middleware.

The middleware parses multipart/form-data and makes file info available to the route.

Conceptual route:

app.post('/upload', requireAuth, upload.single('avatar'), (req, res) => {
  res.json({
    fileName: req.file.filename,
  })
})

Storage options

Files may be stored:

• on local disk,
• in cloud object storage,
• temporarily before processing.

For production, object storage is usually better than storing user uploads inside the app server filesystem.

Common mistake

Do not trust file extension alone.

Validate file size and type, and never allow uploaded files to execute as code.

Interview answer

File uploads in Express are usually handled with multipart middleware. A safe upload flow validates file size and type, requires authorization when needed, stores files in a controlled location, handles errors, and avoids trusting user-provided filenames or extensions blindly.