I also think there needs to be a way to prevent that for important attachments. . . . . .
I think that goes along with the request to provide a way to copy an attachment into the Files section. I'd suggest that "copy" mean creating a second link to the data, not a literal copy of the data (so that the copy doesn't cost extra storage). I think that means having something like inode reference counts in Unix filesystems. . . . . .
In this case the presence of the additional link(s) could prevent the attachment from disappearing from the message (no sense breaking that if the group elected to keep the file content).
Could the file move rather than copy, but carry the links along with it? Once it's out of the folder that deletes files, would it be saved indefinitely? (Yeah, I know, this sounds like a real non-tech statement.)