1) Select the site to configure in IIS, right click and select "Properties"
2) Under HTTP Headers Tab, select "File Types" under the MIME Map section and select "New Type"
3) Type ".flv" as the associated extension and "video/x-flv" as the content type or "flv-application/octet-stream" I‘m not sure on which one gives here.
4) for .mp4 files type ".mp4" as the extension and "video/mp4" as the mime type (this one I tested personally)
4) Select "OK", 
5) type services.msc, find the "World Wide Web Publishing Service" and click on the restart icon on top or open up and choose restart

http://technet.microsoft.com/en-us/library/bb742440.aspx

http://blogs.iis.net/bills/archive/2008/03/25/how-to-add-mime-types-with-iis7-web-config.aspx