mime 4.1.0: Configure custom MIME types and override standard extensions using a new Mime instance in mime v4

# Notes on `mime` v4 Custom Types and Overrides

## `search_known_solution` Result
`search_known_solution` returned existing sample `sha256:de68283133f08ef6991ed21eb301b843d23fb5fa70dc448438763a111403d62c` ("Look up a MIME type from a filename with mime v4"), which demonstrates default export `mime.getType()` and `mime.getExtension()`. This sample explores a different trap: configuring custom MIME types, constructor isolation, and extension overrides in `mime` v4.

## What a naive model would write instead
A naive model would call `mime.define({ 'text/x-custom': ['custom'] })` directly on the default import or instantiate `new Mime(standardTypes, { 'text/x-custom': ['txt'] })` assuming built-in instances are mutable and constructor arguments automatically overwrite collisions.

## How the wrong version fails
It fails loudly at runtime with `Error: define() not allowed for built-in Mime objects` when calling `mime.define()`, or `Error: "..." conflicts with "...". Pass \`force=true\` to override this definition.` when passing conflicting extensions directly to `new Mime(...)` without calling `instance.define(map, true)`.

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{
  "name": "csx-sample-mime-custom-types",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "mime": "4.1.0"
  }
}

import defaultMime, { Mime } from 'mime';
import standardTypes from 'mime/types/standard.js';
import otherTypes from 'mime/types/other.js';

// In mime v4, the default export is frozen via _freeze(). Calling defaultMime.define() throws:
// Error: define() not allowed for built-in Mime objects. See https://github.com/broofa/mime/blob/main/README.md#custom-mime-instances
export function mutateDefaultMime(typeMap) {
  defaultMime.define(typeMap);
}

// In mime v4, new Mime() instances do NOT implicitly inherit built-in types.
// An instance created without standard/other types only recognizes its explicitly passed entries.
export function createIsolatedMime(customTypes) {
  return new Mime(customTypes);
}

// Overrides for already-registered extensions cannot be passed directly into new Mime(standardTypes, overrides)
// because constructor define calls do not set force=true and will throw on conflicts.
// Overriding an existing extension requires calling instance.define(overrides, true).
export function createCustomMime({ customTypes = {}, overrides = {} } = {}) {
  const instance = new Mime(standardTypes, otherTypes, customTypes);
  if (Object.keys(overrides).length > 0) {
    instance.define(overrides, true);
  }
  return instance;
}

export function tryConflictingConstructor(conflictingTypes) {
  return new Mime(standardTypes, conflictingTypes);
}

import { strict as assert } from 'node:assert';
import {
  mutateDefaultMime,
  createIsolatedMime,
  createCustomMime,
  tryConflictingConstructor,
} from '../src/index.mjs';

// 1. Default mime instance is frozen: define() throws in v4
assert.throws(
  () => mutateDefaultMime({ 'application/x-custom-doc': ['custdoc'] }),
  /define\(\) not allowed for built-in Mime objects/
);

// 2. Custom Mime instances without standard types only resolve custom entries
const isolated = createIsolatedMime({ 'application/x-custom-doc': ['custdoc'] });
assert.equal(isolated.getType('report.custdoc'), 'application/x-custom-doc');
assert.equal(isolated.getType('index.html'), null);
assert.equal(isolated.getType('data.json'), null);

// 3. Passing conflicting extension mappings to Mime constructor throws without force=true
assert.throws(
  () => tryConflictingConstructor({ 'text/x-custom-plain': ['txt'] }),
  /conflicts with "text\/plain -> txt"\. Pass `force=true` to override this definition\./
);

// 4. Overriding with define(map, true) on custom instance works alongside standard and other types
const customMime = createCustomMime({
  customTypes: { 'application/x-custom-doc': ['custdoc'] },
  overrides: { 'text/x-custom-plain': ['txt'] },
});

assert.equal(customMime.getType('report.custdoc'), 'application/x-custom-doc');
assert.equal(customMime.getType('notes.txt'), 'text/x-custom-plain');
assert.equal(customMime.getType('data.json'), 'application/json');
assert.equal(customMime.getType('archive.7z'), 'application/x-7z-compressed');
assert.equal(customMime.getType('mystery.zzz'), null);

assert.equal(customMime.getExtension('text/x-custom-plain'), 'txt');
assert.equal(customMime.getExtension('application/x-custom-doc'), 'custdoc');
assert.equal(customMime.getExtension('application/json'), 'json');

console.log('CONTRACT PASS: mime v4 custom Mime instances, dictionary imports, and forced overrides verified');