function extnamesToRegex(extnames) {
  return new RegExp(
    '\\.(' +
      extnames
        .map(function (d) {
          return d.slice(1)
        })
        .join('|') +
      ')([?#]|$)'
  )
}
Turn a list of extnames (*with* dots) into an expression.

@param {ReadonlyArray<string>} extnames
  List of extnames.
@returns {RegExp}
  Regex matching them.