Media: Fix update_attached_file() stripping backslashes from Windows directory separators

[shrivastavanolo]

On Windows, PHP's realpath() returns paths with native backslash directory separators (e.g. C:\WWW\Sites\demo\wp-content\uploads\2016\03\example.jpg). When these paths are passed to update_attached_file(), the backslashes are stripped by wp_unslash() inside update_metadata(), resulting in an invalid path being stored in postmeta (e.g. C:WWWSitesdemowp-contentuploads201603example.jpg).

This breaks the media library entirely on Windows, as file_exists() and other file functions fail on the corrupted path.

The fix passes $file through wp_normalize_path() after _wp_relative_upload_path() converts it to a relative path, ensuring all backslashes are converted to forward slashes before the value is stored in the database.

Trac ticket: https://core.trac.wordpress.org/ticket/36273

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Opus 4.6
Used for: Initial code skeleton; final implementation was reviewed and edited by me.

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