CVE-2026-39844: NiceGUI: Upload filename sanitization bypass via backslashes allows path traversal on Windows

The upload filename sanitization introduced in GHSA-9ffm-fxg3-xrhh uses PurePosixPath(filename).name to strip path components. Since PurePosixPath only recognizes forward slashes (/) as path separators, an attacker can bypass this sanitization on Windows by using backslashes (\) in the upload filename.

Applications that construct file paths using file.name (a pattern demonstrated in NiceGUI’s bundled examples) are vulnerable to arbitrary file write on Windows.