The Recycle Bin has an "automatic cleanup" feature. The number of days documents (and attachments) are kept in the recycle bin can be configured per site.
For example, in site A you can say that documents are automatically deleted every 5 days, whereas in site B documents never automatically deleted.
In the original implementation of this feature, due to our attachments architecture, this didn't work properly for attachments and as soon as one site had this feature, the cleanup task would clean the attachments in all the sites ignoring this setting. Now it should work properly.