mime 4.1.0: Define custom MIME types and override mappings using Mime instance in mime v4

# Notes

## What `search_known_solution` answered
Search returned an existing sample `sha256:de68283133f08ef6991ed21eb301b843d23fb5fa70dc448438763a111403d62c` ("Look up a MIME type from a filename with mime v4") which covers basic `mime.getType` and `mime.getExtension` default lookups.

## What a model would have written instead
A model would call `mime.define({ 'custom/type': ['ext'] })` directly on the imported default `mime` object as in `mime@2`/`mime@3`, expect `mime.getAllExtensions()` to return an `Array`, and register existing extensions without passing `force = true` or `*ext`.

## How the wrong version fails
Fails loudly at runtime with `Error: define() not allowed for built-in Mime objects. See https://github.com/broofa/mime/blob/main/README.md#custom-mime-instances` when calling `define()` on the default export, throws collision errors on existing extensions unless forced, and throws `TypeError` when calling Array methods on the `Set` returned by `getAllExtensions()`.

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{
  "name": "csx-sample-mime-custom-instance",
  "version": "1.0.0",
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  "dependencies": {
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import mime, { Mime } from 'mime';
import standardTypes from 'mime/types/standard.js';

/**
 * Creates a custom Mime registry based on standard types.
 * In mime v4, the default `mime` instance is frozen; calling mime.define() throws.
 * Custom types must be registered on a new Mime instance.
 */
export function createCustomMimeRegistry() {
  return new Mime(standardTypes);
}

/**
 * Registers custom type mappings on a Mime instance.
 * If force is false and an extension is already defined, define() will throw.
 */
export function registerCustomTypes(mimeInstance, typeMap, force = false) {
  return mimeInstance.define(typeMap, force);
}

export default mime;
export { mime, Mime, standardTypes };

import { strict as assert } from 'node:assert';
import mime, { Mime, createCustomMimeRegistry, registerCustomTypes } from '../src/index.mjs';
import standardTypes from 'mime/types/standard.js';

// 1. Default mime instance is frozen in v4 — calling mime.define() throws
assert.throws(
  () => {
    mime.define({ 'application/x-custom': ['custom'] });
  },
  {
    name: 'Error',
    message: /define\(\) not allowed for built-in Mime objects/
  },
  'Calling define() on the default mime export in v4 must throw'
);

// 2. Custom Mime instances allow registering new MIME types
const customRegistry = createCustomMimeRegistry();
registerCustomTypes(customRegistry, { 'application/x-custom-doc': ['cdoc'] });
assert.equal(customRegistry.getType('file.cdoc'), 'application/x-custom-doc');
assert.equal(customRegistry.getExtension('application/x-custom-doc'), 'cdoc');

// 3. Redefining an existing extension throws unless force=true is specified
assert.throws(
  () => {
    // 'json' is already mapped to 'application/json' in standardTypes
    registerCustomTypes(customRegistry, { 'application/x-custom-json': ['json'] }, false);
  },
  {
    name: 'Error',
    message: /conflicts with .* Pass `force=true` to override/
  },
  'Redefining an existing extension without force=true must throw'
);

// With force=true, the mapping is overridden
registerCustomTypes(customRegistry, { 'application/x-custom-json': ['json'] }, true);
assert.equal(customRegistry.getType('payload.json'), 'application/x-custom-json');

// 4. Starred extensions (*ext) allow reverse mapping without overriding forward lookup
const starredRegistry = new Mime(standardTypes);
starredRegistry.define({ 'application/vnd.custom-wasm': ['*wasm', 'cwasm'] });
// Forward lookup for 'wasm' still resolves to original standard type
assert.equal(starredRegistry.getType('module.wasm'), 'application/wasm');
// Forward lookup for unstarred alias resolves to new type
assert.equal(starredRegistry.getType('module.cwasm'), 'application/vnd.custom-wasm');
// Reverse lookup resolves to default (first) extension
assert.equal(starredRegistry.getExtension('application/vnd.custom-wasm'), 'wasm');

// 5. getAllExtensions returns a Set instance (not an Array), or null for unknown
const htmlExts = customRegistry.getAllExtensions('text/html');
assert(htmlExts instanceof Set, 'getAllExtensions must return a Set instance');
assert.equal(Array.isArray(htmlExts), false, 'getAllExtensions must not return an Array');
assert(htmlExts.has('html') && htmlExts.has('htm'));
assert.equal(customRegistry.getAllExtensions('nonexistent/type'), null);

console.log('CONTRACT PASS: custom Mime instance, define conflict rules, and getAllExtensions verified');