Attachment modifications don't appear in the activity stream anymore

It was lost by the move to a new activity stream system in XWIKI-15660/XWIKI-15665.

We do know that a modification was made to the document (since uploading an attachment also cause a document save), we just don't know it's an attachment related change anymore. So not making it a block for this very old regression.

The root cause seems to be that no descriptor was implemented for attachment events (but there is one for component events, for example).