TestCase.create_file_attachment has a has_file argument that allows

creating the attachment without an actual file object (not previously in

the direct signature, but inherited from create_file_attachment_base).

Actually using that would cause a crash when attempting to populate the

sha256 checksum.
This change explicitly lists that argument in the signature and then

uses it to guard against trying to populate the sha256.

Ran unit tests in an extension that used

create_file_attachment(has_file=False) and saw that it no longer

failed.