code: @OverrideOnDemand
    @Nullable
    protected String determineMimeType (@Nonnull final String sFilename, @Nonnull final IReadableResource aResource)
    {
      return MimeTypeInfoManager.getDefaultInstance ().getPrimaryMimeTypeStringForFilename (sFilename);
    }
docs: Determine the MIME type of the resource to deliver.
  
  @param sFilename
  The passed filename to stream. Never <code>null</code>.
  @param aResource
  The resolved resource. Never <code>null</code>.
  @return <code>null</code> if no MIME type could be determined