Gross! When was this put in?
I don't think this is necessary. It can be fixed with a few trivial changes
to the BNF; specifically, moving the required CRLF to the end of a body part
rather than to the front of a boundary, that is (with a few other changes):
close-delimiter := "--" boundary "--" CRLF
delimiter := "--" boundary CRLF
epilogue := optional-text
encapsulation := delimiter part-encapsulation CRLF
multipart-body := preamble 1*encapsulation close-delimiter epilogue
premble := optional-text
optional-text := *[*text CRLF]
-- Mark --