The mp4 extension is valid only for files that will be uploaded after you enable original extension mapping, otherwise the file is processed with sflb.ashx extension.
Another option is extending JWplayer script to include sflb.ashx extension. This should work, because we do not change the mime type of the file.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items