In [38625], the functionality to search for attachments by filename was added via the `posts_clauses` filter and the `_filter_query_attachment_filenames()` function. This moves `_filter_query_attachment_filenames()` from `wp-admin/includes/post.php` to `wp-includes/post.php` so that it can be applied in the same manner in the REST API media endpoint.
Merge of [39598] to the 4.7 branch.
Props jblz, tyxla.
Fixes#39092.
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