function determineContentType (filePath) {
  switch (path.extname(filePath)) {
    case '.html': return 'text/html'
    case '.css': return 'text/css'
    case '.js': return 'application/javascript'
    case '.png': return 'image/png'
    case '.jpg': case '.jpeg': return 'image/jpeg'
    default: return 'application/octet-stream'
  }
}
Determines the correct content type for a file, using the extension in the file's (absolute or relative) path.

By default, s3 sets the content type to application/octet-stream, which results in browsers downloading the assets
instead of serving them as a webpage.

@param {string} filePath
@returns {string} ContentType corresponding to the file extension in the file path provided