File type validations in expressjs/multer

expressjs/multer is a great Node.js middleware which handles the file upload with validations like:

• Number of files to be uploaded
• Size of the files
• File extensions

Sometimes we want to validate the file extension, and also ensure the file format is indeed matching its extension. To achieve this, we can utilise sindresorhus/file-type

const express = require('express');
const multer = require('multer');
const fileType = require('file-type');

const app = new express();
const accepted_extensions = ['jpg', 'png', 'gif'];
const upload = multer({
    limits: { 
        fileSize: 5 * 1024 * 1024,  // 5 MB upload limit
        files: 1                    // 1 file
    },
    fileFilter: (req, file, cb) => {
        // if the file extension is in our accepted list
        if (accepted_extensions.some(ext => file.originalname.endsWith("." + ext))) {
            return cb(null, true);
        }

        // otherwise, return error
        return cb(new Error('Only ' + accepted_extensions.join(", ") + ' files are allowed!'));
    }
});


/**
 * Middleware for validating file format
 */
function validate_format(req, res, next) {
    // For MemoryStorage, validate the format using `req.file.buffer`
    // For DiskStorage, validate the format using `fs.readFile(req.file.path)` from Node.js File System Module
    let mime = fileType(req.file.buffer);

    // if can't be determined or format not accepted
    if(!mime || !accepted_extensions.includes(mime.ext))
        return next(new Error('The uploaded file is not in ' + accepted_extensions.join(", ") + ' format!'));
    
    next();
}

app.post("/", 
    // First middleware, validate number of files (one file) / size (5MB) / extension ('jpg', 'png', 'gif')
    upload.single("file_field_name"), 
    
    // Second middleware, validate file format ('jpg', 'png', 'gif')
    validate_format,

    function(req, res, next) {
        // Passed validations, do something with the file (`req.file`)
        // ...

        
        res.send("file uploaded");
    }
);

app.listen(3000, () => {
    console.log("Express listening on port 3000");
});