GHSA-wg2q-39h6-66x9

## Summary

The multipart upload filename fix splits on the path separator but never rejects dot-dot, allowing a write outside the served tree.

## Finding (Medium): upload filename escapes the served tree (residual of CVE-2026-35393)

The multipart filename fix (updown.go lines 135-136) splits on the path separator but never rejects "..". Uploading with filename=".." results in os.Create against the parent of the upload folder with a trailing marker character, outside the served tree, and the subsequent failed rename leaves that file behind. Verified: a file containing ESCAPED_WRITE_PROOF was written outside the webroot, unauthenticated, with the default configuration. Not claimed: a Windows-specific variant (Go's Part.FileName() already applies filepath.Base).

## Suggested fixes

1. Reject any upload filename that is "..", is empty after sanitisation, or resolves outside the upload folder; validate the final resolved destination rather than only transforming the input.

## Tooling

AI assistance was used while investigating. The finding was reproduced against a running server on loopback.