docs: def sanitize_mimetype(mimetype, filename=None):
      """Sanitize a MIME type so the browser does not render the file."""
      # Allow some few mime type like plain text, images and audio.
      if mimetype in MIMETYPE_WHITELIST:
          return mimetype
      # Rewrite HTML, JavaScript, CSS etc to text/plain.
      if mimetype in MIMETYPE_PLAINTEXT or \
              (filename and filename.lower() in MIMETYPE_TEXTFILES):
          return 'text/plain'
      # Default
      return 'application/octet-stream'