Flex端代码：

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
			   xmlns:s="library://ns.adobe.com/flex/spark" creationComplete="windowedapplication1_creationCompleteHandler(event)" 
			   xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600">
	<fx:Script>
		<![CDATA[
			import mx.controls.Alert;
			import mx.events.FlexEvent;
			import mx.managers.PopUpManager;
			[Bindable]
			private var stateText:String="文件名：";
			[Bindable]
			private var isShow:Boolean=false;
			private var isSelect:Boolean=false;
	
			private var file:FileReference = new FileReference();
			protected function windowedapplication1_creationCompleteHandler(event:FlexEvent):void
			{
				file.addEventListener(Event.SELECT, file_select);
				file.addEventListener(Event.COMPLETE, file_complete);
				file.addEventListener(ProgressEvent.PROGRESS, file_progress);
			}
			
		
			private function file_select (e:Event):void {
				stateText = "文件名： "   file.name;
				isSelect=true;
				isShow=true;
			}
			
			private function file_complete (e:Event):void {
				stateText = "上传完毕";
				PopUpManager.removePopUp(this);
				mtxt.text="Images/FileImage/" file.name;
			}
			
			private function file_progress (e:ProgressEvent):void {
				stateText = "已上传 "   Math.round(100 * e.bytesLoaded / e.bytesTotal)   "%";
			}
			
			private function upload ():void {
				if(isSelect==false){
					stateText="请先选择文件";
					return;
				}
				if (file.size > 0) {
					stateText = "正在上传 "   file.name;
						var request:URLRequest = new URLRequest("../../LegendsFileHandler.aspx");
						file.upload(request);	
				}
			}
			
		]]>
	</fx:Script>
	<s:Group left="10" top="10">
		<s:Label left="10" top="10" text="{stateText}"/>
		<s:Button left="10" top="30" label="选择" click="file.browse();"/>
		<s:Button left="110" top="30" label="上传" visible="{isShow}" click="upload();"/>
		<s:TextInput width="100%" top="50" id="mtxt"/>
	</s:Group>
</s:Application>

ASP.NET端代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace MieMie
{
    public partial class LegendsFileHandler : System.Web.UI.Page
    {
        string uploadFolder = "Image/"; 

        protected void Page_Load(object sender, EventArgs e)
        {
            HttpFileCollection files = Request.Files;

            if (files.Count == 0)
            {
                Response.End();
            }

            string path = Server.MapPath(uploadFolder);

            HttpPostedFile file = files[0];

            if (file != null && file.ContentLength > 0)
            {
                string savePath = path   "/"   Request.Form["fileName"];
                file.SaveAs(savePath);
            }
        }
    }
}

实例下载：http://www.pengyaou.com/LegendsZ/eg/asp.netFlex.rar