File type validation in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is file type validation in Express?

File type validation in Express means checking the type of a file uploaded by a user to make sure it matches allowed formats before saving or processing it.

beginner

Which middleware is commonly used in Express to handle file uploads and validate file types?

Multer is a popular middleware for handling file uploads in Express. It can be configured to validate file types by checking the file's mimetype or extension.

intermediate

How can you reject files with disallowed types using Multer?

You can use Multer's fileFilter option, a function that receives the file and a callback. Inside, check the file's mimetype or extension. If allowed, call callback(null, true); if not, call callback(new Error('Invalid file type'), false).

beginner

Why is it important to validate file types on the server side?

Validating file types on the server protects your app from harmful files, prevents unexpected errors, and ensures users upload only supported formats.

intermediate

What is a simple example of a fileFilter function in Multer that only allows PNG and JPEG images?

A fileFilter function checks if file.mimetype is 'image/png' or 'image/jpeg'. If yes, it accepts the file; otherwise, it rejects it with an error.

Which Express middleware is best suited for handling file uploads and validating file types?

A Body-parser

B Helmet

C Cors

D Multer

Multer is designed for handling multipart/form-data, which is used for file uploads, and supports file type validation.

In Multer's fileFilter function, what should you do to reject a file with an invalid type?

A Call callback(null, true)

B Call callback(new Error('Invalid file type'), false)

C Throw an exception

D Return false

Calling callback with an error and false tells Multer to reject the file and report the error.

Which property of the uploaded file is commonly checked to validate its type?

A file.size

B file.path

C file.mimetype

D file.name

file.mimetype contains the MIME type of the file, like 'image/png', which is used to validate file types.

Why should file type validation not rely only on the file extension?

A Extensions can be changed by users and may not reflect the real file type

B Extensions are always correct

C Extensions are checked by the browser

D Extensions are not sent in uploads

Users can rename files to have any extension, so checking mimetype is more reliable.

What happens if you do not validate file types on the server?

A Users can upload harmful or unsupported files

B All files are accepted safely

C Files are automatically converted

D Uploads are blocked

Without validation, users might upload harmful files or files your app cannot handle.

Explain how to implement file type validation in Express using Multer.

Think about how Multer lets you control which files to accept.

Why is server-side file type validation important even if the client validates files?

Consider what happens if a user ignores client checks.