feat(infer): resolve known content types via mimetype instead of no-op

[nao1215]

Summary

multipartkit/infer.content_type_from_filename and content_type_from_bytes were default no-ops that returned None for every input, including well-known types ("a.png", the PNG magic header, etc.). This made the public infer helpers surprising: a caller asking for the content type of photo.png got None. Issue #59 asks for real inference; the chosen resolution (option 1) is to delegate to the sibling nao1215/mimetype package, which already maintains the IANA + magic-byte tables.

This reverses the v0.13.0 (#52) decision to ship these helpers as documented no-ops. That decision was pinned by test/regression_infer_default_test.gleam; this PR rewrites that file to assert the new contract.

Changes

content_type_from_filename and content_type_from_bytes now delegate to mimetype.filename_to_mime_type_strict / mimetype.detect_strict, returning Some(mimetype.essence_of(_)) for recognised input and None otherwise (option.from_result |> option.map(essence_of)). nao1215/mimetype is added as a runtime dependency (>= 0.24.0 and < 1.0.0).

Inference into the form builder stays opt-in. infer.default_inferer() is intentionally kept a no-op, so add_file_auto still falls through to application/octet-stream and never changes a content type implicitly. A new infer.builtin_inferer() wires the two helpers into an Inferer, so callers can opt in with form.add_file_auto_with(form, ..., infer.builtin_inferer()) without writing the mimetype wiring themselves.

test/regression_infer_default_test.gleam is rewritten to assert: the #59 DoD (known extensions and magic-byte signatures resolve to the expected MIME strings; unknown / empty input is None), that default_inferer() is still a no-op, and that builtin_inferer() resolves.

Docstrings, the README feature bullet, and the CHANGELOG are updated.

Design Decisions

Option 1 (delegate to mimetype) was chosen over option 2 (inline a built-in table) because an inline table would duplicate, and inevitably drift from, the table that the sibling mimetype package already maintains — the very duplication the infer docstrings already steered callers away from. Delegating gives full IANA + magic-byte coverage with no duplicate maintenance.

default_inferer() is deliberately left a no-op rather than wired to mimetype, so add_file_auto's behaviour is unchanged and inference remains an explicit opt-in (builtin_inferer() or a custom Inferer). This keeps the form builder from silently changing content types when the dependency resolves an extension.

Notes

This PR also restores the ## [0.13.0] - 2026-05-20 CHANGELOG header, which was accidentally dropped in #60 — without it, the released 0.13.0 entries (#52, #51, #50) were sitting under [Unreleased].

mimetype.detect_strict classifies arbitrary printable-ASCII input as text/plain; this is reachable only through the direct helper / builtin_inferer (never through add_file_auto), and is documented on content_type_from_bytes.

Closes #59

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Configuration used: defaults

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Run ID: 9ccf476f-5ec5-4220-9fb8-7f63a8ceff5d

📥 Commits

Reviewing files that changed from the base of the PR and between b18c0ed and b5d4e0b.

📒 Files selected for processing (7)

• CHANGELOG.md
• README.md
• gleam.toml
• manifest.toml
• src/multipartkit/form.gleam
• src/multipartkit/infer.gleam
• test/regression_infer_default_test.gleam

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-59-infer-builtin-mimetype

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