To make iis server flv files you need to add the .flv extension to the minetypes for the website.
1. Open the site to configure in IIS. Right click and select “Properties”
2. Click the HTTP Headers Tab, select “File Types” under the MIME Map section, and then click “New Type”. Enter the following:
* Associated Extension box: .flv
* MIME Type box: flv-application/octet-stream
3. Click “OK” and close the IIS Properties box
4. You may need to restart the WWW Publishing Service