说简单点也就是
在iis里面http头中加一个
MIME类型 关联扩展名称取.flv
内容类型取 flv-application/octet-stream

操作步骤为：打开IIS－>找到站点->右键属性->http头页项->MIME类型->点击新建->扩展名填写：.flv->MIME类型为：flv-application/octet-stream  如下图：
 

 参考内容：

Windows 2003 Server does not stream FLV videos

Please be aware that these steps do not resolve any issue with Macromedia Flash, but are a configuration step for Microsoft Windows 2003 and Microsoft IIS Server 6.0. Any difficulties in executing those instructions or any errors that may arise from modifying your system settings should be addressed to Microsoft. For more details, please refer to your IIS documentation.

On the Windows 2003 server, open the Internet Information Services Manager.
Expand the Local Computer Server.
Right-click the local computer server and select Properties.
Select the MIME Types tab.
Click the New... button and enter the following information:

Associated Extension box: .FLV
MIME Type box: flv-application/octet-stream

Click Ok.
Restart the World Wide Web Publishing service.