old gem solution

gistfile1.rb

# You could do it before, after save or in any place you want
before_save do   
  # build i file if you want to check something with it first
  file = @api.file(self."#{file_attribute}")
  if file.something ...
    url = file.cdn_url
  end
  # or use bare url widget passed to you from web form
  url = self."#{file_attribute}"
  
  # it is important to wrap it up in case remote call fails for any reason
  begin     
    response = @api.request :post, "/files/", {:source => url, :target => "#{s3_target_url}"}
    # {
    #   "url" : "http://..." 
    # }
    # do that you must. it is basicly JSON string.
  rescue
    # in case everything fails. 
    # it is not happens very ofter but your application can not depends on server status of any other server, right?
    # you may put it to query to call again in five minutes in separate process for example     
    logger.error "#{error text}"   
  end  
end