Signing fail if Thumbs.db exist

If a user accesses their user content folder by Windows Explorer, `Thumbs.db` will be created. Then if site owner didn't add `Thumbs.db` to "ignore" node in `content.json` and only allow specified files, signing won't success.
The similar problems are in `desktop.ini` and `.DS_Store`.

Start by tracing the signing and file-allowlist handling for content.json, then reproduce the failure with Thumbs.db, desktop.ini, and .DS_Store present. Done means the signing behavior is defined and verified for these Windows and macOS-generated files without requiring each one to be manually added.

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