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Resolve and de-duplicate attachment file names: last extension suffix · case 04

The copy counter is inserted before the first dot, splitting multi-part extensions differently from the contract.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Each part has optional fstar (already decoded filename*), fname (filename) and ctname (Content-Type name) and a type. Precedence is fstar, fname, ctname. Directory components are removed (both "/" and "\" separate). An empty name, "." or ".." becomes "attachment" plus an extension by type (.pdf, .png, .txt, else .bin). Names are de-duplicated case-insensitively: the k-th copy gets " (k)" before the last extension, or at the end when there is no extension or the name starts with its only dot.

Why this case matters

Attachment names end up on disk; wrong names overwrite files, leak paths or confuse users.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
multi-dot duplicate	`["archive.tar.gz", "archive (2).tar.gz"]`	`["archive.tar.gz", "archive.tar (2).gz"]`	Failed

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