Mp4 is displaying as Text

Hi, 

We have 2 SharePoint Farms. One Live, second Development. We have Mp4 uploaded to the libraries. The problem is then we try to download Mp4 file on Live we get Text in the browser. But if we do the same on Development we are able to save and download the file. 

Both does NOT have MIME types configured (server or site level).
Same Windows Server 2008 R2.
Same SharePoint Server 2010 with SP2.
Same site collections (just different URLs).
Same services configuration.

I know that this can be solved by adding MIME type, but why we can open MP4 files in Development without MIME types?