report/download and export/download (any download key), the log-file routes and
backup download built a path from the file name in the URL with Path.Combine
and no check, so on Windows a name with a folder part or an absolute path could
reach files outside the folder (security review F21). New helper
FileUtil.FullPathInFolderOrNull accepts only a bare file name that resolves
directly inside the folder; TemporaryFileExists and BackupFileExists use it
(the download routes already call them first), and so do both log routes.
A refused name gets the same 404 as a missing file.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
