本來呢個問題應該好簡單~
基本上一個 ActionScript developer 可以即時答得出

var t_file_ref:FileReference = new FileReference ();
var t_url_request:URLRequest = new URLRequest ("some_path/some_file.type");
t_file_ref.download (t_url_request);

但係原來o係某 d 情況佢係會 malfunction 既…
而且係你估都估唔到~ 加上玩足你幾個鐘…

為免大家中招所以寫左呢個 example

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睇落好似無分別~就算 click “這個不能下載” 都出現左下載畫面但係當你以為 download 左既時候…
你會發覺乜都見唔到…其實問題係在於 FileReference o既 instance declaration method如果o係 function
裡面 declare 既 temporary variable當 function 完成之後 FileReference instance 就會o係 select event 之前消失結果… download 左個桔…
解決方法係將 FileReference instance 放o係 class member variable

public class some_class
{
    private var _file_ref:FileReference;
    function some_function ()
    {
        this._file_ref = new FileReference ();
        ...
    }
}

其實呢個問題… 都幾荒謬… 不過可以玩謝好多 developer…Adobe o既 Technote 有詳細解釋~
http://kb.adobe.com/selfservice/viewContent.do?externalId=3637d5c3&sliceId=1

English title: How to download a file in Flex using FileReference

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