SHA-2 type is not "base64": better a string of 64 (SHA-256) hex digits (hexits).

*SHA-2* for ``here is a simple attachment`` is:

``495395e777cd98da653df9615d09c0fd6bb2f8d4788394cd53c56a3bfdcd848a``

as written in the example few lines below while a *Base64* representation of that hash is:

``NDk1Mzk1ZTc3N2NkOThkYTY1M2RmOTYxNWQwOWMwZmQ2YmIyZjhkNDc4ODM5NGNkNTNjNTZhM2JmZGNkODQ4YQ==``

Maybe, that type has been written thinking at a possible encoding method for the binary part of the multipart payload.