Attachments - Checksum Validation Enhancements

[ericenns]

Summary

Currently the AttachmentChecksumValidator does not allow any file to be attached to an attachable when an existing file matches the checksum of the file attempting to be attached. This results in a scenario where 2 separate files with different names but matching checksums cannot be attached to an attachable. We want to change this behaviour to only raise a validation error when an existing file with the same name and checksum are already attached to the attachable.

Acceptance criteria

• don't allow a duplicate file to be attached to an attachable
• allow a file with a different filename to be attached to an attachable even though it matches another file_blob with the same checksum.