code: function createSourceFilesMiddleware (filesPromise, serveFile, basePath, urlRoot) {
    return function (request, response, next) {
      const requestedFilePath = composeUrl(request.url, basePath, urlRoot)
      // When a path contains HTML-encoded characters (e.g %2F used by Jenkins for branches with /)
      const requestedFilePathUnescaped = composeUrl(querystring.unescape(request.url), basePath, urlRoot)
  
      request.pause()
  
      log.debug(`Requesting ${request.url}`)
      log.debug(`Fetching ${requestedFilePath}`)
  
      return filesPromise.then(function (files) {
        // TODO(vojta): change served to be a map rather then an array
        const file = findByPath(files.served, requestedFilePath) || findByPath(files.served, requestedFilePathUnescaped)
        const rangeHeader = request.headers['range']
  
        if (file) {
          const acceptEncodingHeader = request.headers['accept-encoding']
          const matchedEncoding = Object.keys(file.encodings).find(
            (encoding) => new RegExp(`(^|.*, ?)${encoding}(,|$)`).test(acceptEncodingHeader)
          )
          const content = file.encodings[matchedEncoding] || file.content
  
          serveFile(file.contentPath || file.path, rangeHeader, response, function () {
            if (/\?\w+/.test(request.url)) {
              common.setHeavyCacheHeaders(response) // files with timestamps - cache one year, rely on timestamps
            } else {
              common.setNoCacheHeaders(response) // without timestamps - no cache (debug)
            }
            if (matchedEncoding) {
              response.setHeader('Content-Encoding', matchedEncoding)
            }
          }, content, file.doNotCache)
        } else {
          next()
        }
  
        request.resume()
      })
    }
  }
docs: Source Files middleware is responsible for serving all the source files under the test.