Browser-based TNEF parser opens winmail.dat files without uploading them

A developer found that opening Outlook's winmail.dat attachments usually means uploading them to third-party converters, which is risky for private email content. They built a TypeScript TNEF parser that runs entirely in the browser, so the file never leaves the user's machine. The format packs a message's body, attachments and metadata into one binary blob, and the write-up describes three common pitfalls: padding after length-prefixed values, extra headers on named properties, and conflicting or legacy-encoded attachment names. The author also reports two problems found only after release, including bodies that appeared empty and sender addresses stored in a form that cannot receive mail.