So why didn't that customer just start sending emails that weren't MIME compatible--and so wouldn't have "attachments"--but still carried a very large amount of base64 encoded data (which is all MIME is doing anyway)?
As I recall, in this customer's use case, they wanted to deliver the data as email attachments that the recipient could download (recipients were regular people). I think they were selling ringtones or MP3s or something of that nature and the email was the method of delivery. It was a pretty cool concept.