The keys that have correctly signed the whole attachment. Can be empty, for example non-contract attachments won't be necessarily be signed.
The parties that have correctly signed the whole attachment. Even though this returns a list of party objects, it is not required that these parties exist on the network, but rather they are a mapping from the signing key to the X.500 name.
Attachment size in bytes.
Finds the named file case insensitively and copies it to the output stream.