Media: Make sure `attachment_url_to_postid()` performs a case-sensitive search...

Media: Make sure `attachment_url_to_postid()` performs a case-sensitive search for the uploaded file name.

Previously, the first available match was returned, regardless of the case, which was not always the expected result.

Props archon810, ben.greeley, tristangemus, vsamoletov, SergeyBiryukov.
Fixes #39768.
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git-svn-id: https://core.svn.wordpress.org/trunk@46810 1a063a9b-81f0-0310-95a4-ce76da25c4cd