Is it possible to add an attachment to a form library?
I tried publishing a form that has an attachment control but getting "The selected field cannot be promoted because its data type is not supported: base64binary"
No, it is not possible - you will have to store (add/remove) the attachements in a different document library. You'll have to do the adding/removing yourself - either with custom code (in your InfoPath Form or better in a custom WebService) or OOTB WebServices.
Here is a good article about the Top 10 Questions about InfoPath File Attachements. See Point Nr. 5 for uploading to different document libraries.
It appears Base64Binary is not allowed as a promoted property. You need to write custom code to upload the file to the Document Library instead of as an attachment control.