Turn "HTML" files into JS and CSS TextOutputs

assets.js

function unescape(content){
  return content.replace('&lt;', '<')
                .replace('&gt;', '>');
}

function getJavaScriptTextOutput(filename){
  var content = HtmlService.createHtmlOutputFromFile(filename).getContent();
  content = unescape(content);
  
  return ContentService.createTextOutput( content ).setMimeType(ContentService.MimeType.JAVASCRIPT);
}

function getCSSTextOutput(filename){
  var content = HtmlService.createHtmlOutputFromFile(filename).getContent();
  content = unescape(content);
  var textOutput =ContentService.createTextOutput( content );
  return ContentService.createTextOutput( content ).setMimeType(ContentService.MimeType.CSS);
}

doGetFiltering.js

function doGet(req) {
  "use strict";
  
  var parameters = req.parameter;

  // <script src="?script=file.js"></script>
  if(parameters.script){
    return getJavaScriptTextOutput(parameters.script);
  }
  
  // <link rel="stylesheet" href="?style=file.css">
  if(parameters.style){
    return getCSSTextOutput(parameters.style)
  }

  // not an asset. Do what you would normally.
}

Doesn't seem that it works given that ContentService.MimeType.CSS is undefined.