string outputDirectory = @"C:\output";
using (Viewer viewer = new Viewer("with_attachments.msg"))
{
    IList<Attachment> attachments = viewer.GetAttachments();
    foreach(Attachment attachment in attachments)
    {
        string filePath = Path.Combine(outputDirectory, attachment.FileName);
        viewer.SaveAttachment(attachment.Id, File.OpenWrite(filePath));
    }
}