>>Don't know. Blat must have some option for content type.
>
>Well, here are my options. Anything standout at you?
>
> : file with the message body ('-' for console input, end with ^Z)
>-x : Custom 'X-' header. eg: -x "X-INFO: Blat is Great!"

Perhaps x-header will let you put the content type in there. What about blat technical support?