docs: @Nonnull
    public FineUploader5Validation addAllowedExtension (@Nonnull @Nonempty final String sAllowedExtension)
    {
      ValueEnforcer.notEmpty (sAllowedExtension, "AllowedExtension");
      m_aValidationAllowedExtensions.add (sAllowedExtension);
      return this;
    }