code: @Nonnull
    public FineUploaderBasic addCustomHeader (@Nonnull @Nonempty final String sKey, @Nonnull final String sValue)
    {
      ValueEnforcer.notEmpty (sKey, "Key");
      ValueEnforcer.notNull (sValue, "Value");
  
      m_aRequestCustomHeaders.put (sKey, sValue);
      return this;
    }
docs: Additional headers sent along with the XHR POST request. Note that is option
  is only relevant to the ajax/XHR uploader.
  
  @param sKey
  Custom header name
  @param sValue
  Custom header value
  @return this