I ran into a problem of not being able to play FLV on one of the servers. After talking with some experts and thorough research, Here is the solution:

Adding .flv MIME type in IIS

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.
4) Click “OK”. DONE!