Closes#21575.
## Summary
`getFileNameAndExtension` split filenames with `lastIndexOf('.')`. For
an extensionless name (`README`, `Makefile`), `lastIndexOf` returns `-1`
and `substring`'s negative-index clamping returned `{ name: '',
extension: 'README' }` — losing the whole name into the extension. In
the attachments UI this rendered an empty rename field and corrupted the
name on edit.
## Changes
- Early return `{ name, extension: '' }` when there is no dot
- Correct the existing test that asserted the buggy output; add no-dot +
trailing-dot cases
## Testing
Pure, dependency-free function — verified deterministically (17/17
assertions, red-green proven: reverting the fix fails the corrected
`README` assertion). Note: twenty's full nx lint/test wasn't run locally
(repo wants Node ^24.5.0; this box is on Node 26), so CI is the
authoritative check for lint/format.
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Yash Singh
d7d4b36d5e
