You are correct, and the index table does work this way, and is the main reason that it was created.
The issue is that the attachments search via the index table engine is not yet implemented, it is under construction though. Initially it was available, but some unexpected issues came up regarding attachments type results and we had to remove it and use the default engine instead.
Since it never came up as a request since then, the problem was marked low priority feature. I have bumped it up to a high priority issue, and will try to come up with a solution within the next 2 releases, if possible.
Best,Ernest Marcinko
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