A new function `wp_filesize()` was added with [52837]. The function lived in the `wp-admin/includes/file.php` file. However, this admin specific function is not loaded into memory when hitting `media/edit` endpoint. The result was a `500` Internal Server Error. Why? The function is invoked with that endpoint, but the function does not exist in memory.
This commit relocates the new function to the `wp-includes/functions.php` file. In doing so, the function is available for both the frontend and backend.
Follow-up to [52837].
Props talldanwp, spacedmonkey, costdev, antonvlasenko.
Fixes#55367.
Built from https://develop.svn.wordpress.org/trunk@52932
git-svn-id: http://core.svn.wordpress.org/trunk@52521 1a063a9b-81f0-0310-95a4-ce76da25c4cd