I had it attached as "other" and typed in the description Form 1116B, but it was rejected.  I tried to change the choice to Form 1116 (wasn't there when I had first attached the 1116b, but was by the time I received the 8879 back). It appeared to change it, but e-file was rejected again.  I had to remove and reattach the pdf and select Form 1116 as type and it worked.

I hope this helps.