File Upload Validation: Size, Type & Extension (Frontend + Backend)

const ALLOWED_MIME_TYPES = ['application/pdf', 'image/jpeg', 'image/png'];
const MAX_FILE_SIZE = 25 * 1024 * 1024; // 25MB

export function validateFileBeforeUpload(file: File): { valid: boolean; error?: string } {
  if (file.size === 0) {
    return { valid: false, error: 'File is empty (0 bytes).' };
  }

  if (file.size > MAX_FILE_SIZE) {
    return {
      valid: false,
      error: `File size (${(file.size / (1024 * 1024)).toFixed(1)}MB) exceeds 25MB limit.`,
    };
  }

  if (!ALLOWED_MIME_TYPES.includes(file.type)) {
    return {
      valid: false,
      error: 'Invalid file format. Only PDF, JPG, and PNG are permitted.',
    };
  }

  return { valid: true };
}

import express from 'express';
import multer from 'multer';

const upload = multer({
  limits: {
    fileSize: 25 * 1024 * 1024, // 25 MB max
    files: 1,
  },
  fileFilter: (req, file, cb) => {
    const allowedTypes = ['application/pdf', 'image/jpeg', 'image/png'];
    if (!allowedTypes.includes(file.mimetype)) {
      return cb(new Error('LIMIT_UNEXPECTED_FILETYPE'));
    }
    cb(null, true);
  },
});

const app = express();

app.post('/api/upload', upload.single('document'), (req, res) => {
  if (!req.file) {
    return res.status(400).json({ error: 'No file uploaded' });
  }

  res.status(200).json({
    message: 'File successfully validated and received',
    filename: req.file.originalname,
    size: req.file.size,
  });
});