Hi,
According to your description, I want to know how to modify the MIME setting. Here is a step of configuring MIME type mapping, please have a try:
Go to Start-> Control Panel-> Administrative Tools-> Internet Information Services (IIS) manager-> right-click the properties of your site-> select MIME types-> click new-> enter “.flv” in the “Extension” box, enter “flv-application/octet-stream” in the “MIME type” box->click OK.
After doing that, you need to make sure whether flv video is able to download.
Best regards
David Hu