Yes,
You need to wrap the document in MIME content-type and section codes.

IIRC newer versions of blat include MIME directives - the type you are looking for is "text/html"

+++Update+++
Found this on the blat home page :
quote:
1.9.4 2001.12.12
Speed up to processing of messages with mime/html/enriched attachments
Add quotes around 'boundary' fields