Hi Bjoern,
I haven't tested this, but my best idea is to use a business rule for the attachment name.
The business rule would have two parameters, workflow id and attachment name. With this you should be able to identify the current attachment and return the new name with ATT_ID.
If this doesn't work for some reason, you could add a flow control step afterwards. A check is added, to verify that all attachments have an id in the name. Otherwise increase a counter (technical field), retrieve the first attachment without an Id, update it's name with the counter and test again. :)
An alternative would be to use an item list, whit linked attachments. Than the users could provide some more information and if the users only need an id to which they can refer, than the item list row id could be fine.
There's one draw back with the first option: If the user adds two attachments with the same name from different folders, this will probably fail. On the other hand, it may work, if the updates are processed sequential, than the first attachment is already renamed when the other ones name is determined.
Off topic:
Is there any chance to move this thread from 'user voice' to 'forum'.
Best regards,
Daniel