I tried to create a serializer for an asset of type file that is part of a PortableText field, in order to display a download link for the file. This is what works for me,...

import React from 'react'
import css from './downloadableFile.module.scss'

export default ({ node }) => {
  const baseURL = '<https://cdn.sanity.io/files/>'
  const fileName = `${node.file.asset._ref.split('-')[1]}.${node.file.asset._ref.split('-')[2]}`
  const downloadURL = `${baseURL}${process.env.SANITY_PROJECT_ID}/${process.env.SANITY_DATASET}/${fileName}?dl`

  return (
    <div className={css.root}>
      <p>{node.title} <a href={`${downloadURL}`}>Download</a></p>
    </div>
  )
}