As mentioned in /r/15164, the M2M field for file attachments and

their review request only gets set after the attachment's first save.

However, it would be useful to access the review request of a file

attachment before the first save. File attachments already made use of

a cached _review_request instance variable that we would set in

BaseReviewRequestDetails.get_file_attachments(). This change similarly

sets the attribute when a file attachment is created, since we have the

review request already. This also improves

FileAttachment.get_review_request() to set the cached instance if not

already set when its called.

Ran unit tests.
Used in a change where we call attachment.get_review_request() in a

  file attachment post-save signal handler upon the first save, saw the

  cached instance was used.