function acceptHeaderExtensions (accept) {
  var exts = []
  var parts = (accept || '').split(',')
  if (parts.includes('text/html') || (parts.length === 1 && parts[0] === '*/*')) exts.push('.html')
  if (parts.includes('text/css')) exts.push('.css')
  if (parts.includes('image/*') || parts.includes('image/apng')) exts = exts.concat(['.png', '.jpg', '.jpeg', '.gif'])
  return exts
}
For a given HTTP accept header, provide a list of file-extensions to try.
@param {string | undefined} accept
@returns {string[]}