The `messageContentForServerDB(...)` function determines what gets put in the `content` column of the `messages` table.
The current `content` column for media messages is a "stringified" list of `uploadID`s. However, the content column is never used after it's been stored in the DB. Rather, messages are joined with corresponding uploads via the `container` column of the `uploads` table. Therefore this change isn't "destructive."