This matches the documentation for the filter of the same name.
Previously, the function could return `false` for an audio or video attachment that does not exist in the local filesystem.
Props Chouby, SergeyBiryukov.
Fixes#52603.
Built from https://develop.svn.wordpress.org/trunk@51162
git-svn-id: http://core.svn.wordpress.org/trunk@50771 1a063a9b-81f0-0310-95a4-ce76da25c4cd