- Save: overwriting an existing file first copies it to a sibling <name>.bak
(default on; toggle with the ISAVEBAK command, persisted in settings).
- Open: .bak is a first-class type — the Open dialog lists it and the loader
detects DWG vs DXF from the file's leading magic bytes (the extension hides
the real format).
- File association registers .bak too, every launch (so users who associated
earlier get it automatically): Windows via the registry; Linux via a
high-priority shared-mime-info magic that resolves DWG/DXF content under the
.bak extension, with the generic *.bak glob claimed only once the app is the
default DWG handler — so it never hijacks other apps' backups.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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