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 (
    &lt;div className={css.root}&gt;
      &lt;p&gt;{node.title} &lt;a href={`${downloadURL}`}&gt;Download&lt;/a&gt;&lt;/p&gt;
    &lt;/div&gt;
  )
}