Some attachment metadata can now be edited, and the is_spoiler parameter should be used for setting spoiler status.
Apps can now edit the description and is_spoiler parameters on existing attachments when editing messages.
We have also separated the attachment request object in the docs to make it more clear which params you can set for attachments and which fields are provided by the API.
Attachments can be marked as a spoiler by setting is_spoiler: true in the attachment request object, and apps can read whether an existing attachment is spoilered by checking for the IS_SPOILER attachment flag on the attachment object. The legacy SPOILER_ filename prefix will continue to mark attachments as a spoiler, but not all spoilered attachments have the prefix.