Close parsed files after each indexing iteration  (fixes #11949) (rebased onto develop)



This is the same as gh-2059 but rebased onto develop.

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To test, attempt to add a file attachment to an image containing some
known string. Wait for indexing of the file to complete (should only be
a few moments if the system is up to date; otherwise check the
Indexer-0.log). Then search for the known string and check that the
image the file was attached to shows up in the search results.