[kernel-838] add fix for cli file upload

[archandatta]

Test:

  # NEW: files.file / files.dest_path (no-index comma format)
  curl -s -w " HTTP %{http_code}" -X POST "$BASE/fs/upload" \
    -F "files.file=@/tmp/curl_test_payload.txt" \
    -F "files.dest_path=/tmp/test_comma_format.txt"

  # Verify it was written
  curl -s -w " HTTP %{http_code}" "$BASE/fs/read_file?path=/tmp/test_comma_format.txt"

---

Note

Medium Risk
Changes multipart field parsing for the filesystem upload endpoint, which could affect how file uploads are interpreted and written to disk if clients send ambiguous field names.

Overview
UploadFiles now accepts a no-index multipart “comma” array format (files.file and files.dest_path), treating it as index 0 in addition to the existing indexed field encodings.

Adds a focused regression test (TestUploadFilesCommaFormat) to verify the new field naming convention successfully uploads and writes the file to the requested destination.

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