Custom field filtering is purposfully disabld for attachments, as it may lead to no results in cases when both posts and media is selected for filtering.
I'm planning to roll out version 4.11.7 later today for approval. What I could do is to perhaps add an option to turn off the custom field filters for attachments, to allow those filters to affect attachments as well. I have noted this as a new feature, keep an eye on updates, it should be available within 3 days time.Best,
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