Narrow the scope of the included wp-admin files loaded for `wp_generate_attachment_metadata()`, `wp_handle_upload()`, `wp_tempnam()`, and `wp_handle_sideload()`. Requires only wp-admin/includes/file.php and wp-admin/includes/image.php instead of wp-admin/includes/admin.php.
Props lonelyvegan, soulseekah, pratikthink.
Fixes#43757.
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