By default, 3GP file extension is not understand and serve by the IIS, hence you might get the error as below when you try to browse to your 3GP file from your IIS web services engine.
The problem is IIS handler mapping do not have 3GP file extension format by default. To get this fixed, you could add the extension through the handler mapping from IIS 7.x.
Alternatively, you may run the following command to add the extension.
%windir%\system32\inetsrv\appcmd set config /section:staticContent /+"[fileExtension='.3gp',mimeType='video/3gpp']"