code: @Nonnull
    public FineUploader5Validation addAllowedExtension (@Nonnull @Nonempty final String sAllowedExtension)
    {
      ValueEnforcer.notEmpty (sAllowedExtension, "AllowedExtension");
      m_aValidationAllowedExtensions.add (sAllowedExtension);
      return this;
    }
docs: Specify file valid file extensions here to restrict uploads to specific
  types.
  
  @param sAllowedExtension
  The allowed extension to be added. E.g. ("jpeg", "jpg", "gif")
  @return this for chaining