`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. Testing Done: Ran unit tests in an extension that used `create_file_attachment(has_file=False)` and saw that it no longer failed.