[bestgopher]

Native Instant uploads add a local Content-Type and then append the same header from the signed upload target. RustFS 1.0.3 rejects these PUTs with 400 InvalidRequest: duplicate header: content-type, leaving manifest, metadata, and thumbnail uploads incomplete even after video segments have uploaded.

Use the server-provided content type when present, matching header names case-insensitively, and infer a MIME type only when it is absent. Extract request construction for regression tests covering header casing, supplied MIME precedence, fallback types, additional headers, request bodies, and Google Drive content ranges.

Validation:

• Two regression tests extracted with the production request builder fail against the original construction and pass with this fix using reqwest 0.12.28.
• Real Cap-signed RustFS PUTs for a native recording's manifest, metadata JSON, and JPEG thumbnail returned 200 OK with the fixed builder.
• Scoped formatting and git diff --check passed.

Full desktop cargo check is blocked by a missing local zed-cap checkout. The complete patched GUI has not yet been rebuilt and retested; the real upload verification used the extracted production request builder.

Closes #3462

Confidence Score: 5/5

The PR appears safe to merge.

Summary

Prevents duplicate Content-Type headers in native signed uploads.

• Signed uploads use the target's content type when it provides one.

No actionable issues found. Review was static; tests were not run.

_{Reviews (1) · Last reviewed commit: "fix(desktop): avoid duplicate content ty..." · Reviewed by Greptile}

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