How to add callback params to new API?

Using the old Rhomobile API, one could add callback params sing callback_params option:

Rho::AsyncHttp.download_file(
    :url => file_url,
    :filename => savefile_name,
    :headers => {"Content-Type"=>"application/octet-stream"},
    :callback => url_for(:action => :download_callback),
    :callback_param => "filename=#{Rho::RhoSupport.url_encode(file_name)}"
)

I have tried the following with new the API, but it does not pass the params:

downloadfileProps = Hash.new
downloadfileProps["url"] = file_url
downloadfileProps["filename"] = savefile_name
Rho::Network.downloadFile(downloadfileProps, url_for(:action => :download_callback, :filename => Rho::RhoSupport.url_encode(file_name) ) )