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`defaultFileName` in `src/filename.ts` now gives the name a file is saved under when the caller gives no path, and `originalName`, `thumbnailName`, `downloadFile` and `downloadThumbnail` all call it. For a live photo's image and video it puts the part's own extension in before cutting the name short, so those names fit too; a path from `--out` or `outPath` is kept as given. `shortenKeepingExtension` keeps the first character before the extension, cutting a long extension short to leave room for it, so a cut name no longer starts with a dot. The backup's links for such a title change to match. TODO.md names the open bug and describes what the code does. Model: opus-5-5