How to use Jest to test file download?

I would mock out FileSaver with a spy:

import FileSaver from 'file-saver';
jest.mock('file-saver', ()=>({saveAs: jest.fn()}))

As you cant compare Blobs I would mock this as well:

global.Blob = function (content, options){return  ({content, options})}

now you can run your test and use expect like this

createDownload('content', 'filename', 'extension')
expect(FileSaver.saveAs).toHaveBeenCalledWith(
  {content:'content', options: { type: 'application/octet-stream' }}, 
  'filename.extension'
)