Expand this.promiser for some attributes in files promises.

Supported are attributes in body classes and log-attributes in
body action, as well as the previously supported transformer.

Addresses the highest priority items left in Redmine #1554.
It does free the buffer (so `NoFree` could mislead), it doesn't free the content.
It needs to convey that it does destroy the buffer, of course.
How about `ExtractBufferDelete` ?  It extracts the content and deletes the buffer.