For supporting flv extension in your web server, there is one settings you should apply. It's really simple. Check the following steps :
- Open IIS manager in your web server.
- Right click on your web site directory in the IIS and click properties.
- Find HTTP Headers tab and click on it.
- Click on MIME Types button.
- Type .flv into Extension textbox then videx/x-flv into MIME Type textbox.
- Finally, click OK button. Now, it's ready.
For more information about other MIME types, you can read the web page below which contains MIME Media Types.