As you can see in RFC 6068, this is not possible at all:
The special
<hfname>“body” indicates that the associated<hfvalue>
is the body of the message. The “body” field value is intended to
contain the content for the first text/plain body part of the
message. The “body” pseudo header field is primarily intended for
the generation of short text messages for automatic processing (such
as “subscribe” messages for mailing lists), not for general MIME
bodies.