REST API: Prevent duplicate firing of `rest(_after)?_insert_attachment` actions.

Merges [43862] from the 5.0 branch to trunk.

Props danielbachhuber, peterwilsoncc.
Fixes #45269.
See #42864.

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git-svn-id: https://core.svn.wordpress.org/trunk@44055 1a063a9b-81f0-0310-95a4-ce76da25c4cd