Changing your code to

        $msg->attach(Type     =>'image/gif',
                    Path=>'/foo/baa.gif',
                    Disposition => 'attachment'
                    )
            ->attr( 'content-type.name', undef );
[download]

works. Here, I am using that attach returns the new part; you're probably more familiar with MIME::Lite than I am, so if you wish to separate this into two lines, you need to find some sort of accessors for parts --- I was unable to find them.

I'm unsure if this is the recommended way of doing this; I think it should be fine.