Why unknown file types are not served by IIS

Here is the command:

%windir%\system32\inetsrv\appcmd set config /section:staticContent /+"[fileExtension='.3gp',mimeType='video/3gpp']"

And while we are at it. You might run into the same problem with MP4 files. Here is the appcmd for it:

%windir%\system32\inetsrv\appcmd set config /section:staticContent /+"[fileExtension='.mp4',mimeType='video/mpeg']"