function createParts(file, partSize = 5 * 1024 * 1024) {  // 5 MB default
  const parts = [];
  const numParts = Math.ceil(file.size / partSize);
  for (let i = 0; i < numParts; i++) {
    const start = i * partSize;
    const end = Math.min(start + partSize, file.size);
    parts.push(file.slice(start, end));
  }
  return parts;
}

// Usage
const fileInput = document.querySelector('input[type="file"]');
const file = fileInput.files[0];
const chunks = createParts(file, 10 * 1024 * 1024);  // 10 MB chunks
console.log(chunks.length);  // e.g., 3 for 25 MB file