Inline CsvPreviewController before_action
Inline CsvPreviewController before_action

This controller only has a single method so it seems simpler to inline
the logic in the `:reject_non_previewable_attachments` `before_action`
into the `preview` method.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall