Summary:
When editing either the filename or the comment of a "News" attachment, Confluence complains that you're trying to move the attachment to a non-existant page and the update then fails.

Description:
The ability to rename attachments and edit attachment comments is great and it seems to be working fine when used with "Pages". However, when you try the same trick with a "News" (the content type formerly known as "Blogs"), it generates the following error:

"You can not move the attachment to a non existing page. Please make sure that the choosen page exists in the selected space"

This occurs when you leave the "Page" field of the Edit Attachments form empty, just as you do with an actual Page when you have no intention of moving the attachment.