Modelform for uploading and/or editing attachments.
Make sure the attachment file size is allowed.
Make sure the honeypot field is empty.
Make sure the security hash match what’s expected.
Make sure the timestamp is not too far (> 2 hours) in the past.
Generate initial security data for the form.
Generate a HMAC security hash from the provided info.
Get the initial security hash from self.content_object and a (unix) timestamp.
Return the errors associated with security.