Add the .rar extension to the AllowedUploadFileExtensions key in web.config/user.config, like this:

<add key="AllowedUploadFileExtensions" value=".cer|.gif|.jpg| ... |.rar">

That will allow you to upload files with the .rar extension using the file browsers in mojoPortal. If users with lower privileges than administrator/content administrator need to upload rar files, you can also add that file extension to the key AllowedLessPriveledgedUserUploadFileExtensions.

You might also need to add to or modify the MIME types defined in IIS. With some quick searching I've seen the MIME type for rar listed as either "application/octet-stream" or "application/x-rar-compressed". You can experiment with that MIME setting across different browsers to figure out what works best for you.

Jamie