S/MIME signing - wrong hash

[Bykiev]

Hi, I'm new to s/mime and need to digitally sign email with xml attachment, but unfortunately this email has a wrong hash value. I digged into the code of the library and found that it creates a sign for base64-encoded body part, is it correct or the signature should be computed for xml attachment content?

Also here is some more issues:

1. Lots of headers/parameters are owerritten by library: for ex. ContentType parameters, some headers (like X-Mailer) and many others
2. It creates an empty boundary for Content-Type: text/plain, though I haven't any text except attachment
3. What is the difference between SmimeEnvelopeAndSignBy() and SmimeAttachSignatureBy()