fix: handle unknown Content-Type in res.set() falling back to original value

[AkaHarshit]

Problem

When calling res.set('Content-Type', value) where the value doesn't contain a / (like a bare extension or shorthand), mime.contentType(value) is used to resolve the full MIME type. However, if mime.contentType() returns false (for unrecognized types), the Content-Type header is set to the literal string "false" instead of keeping the original value.

Reproduction:

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  res.set('Content-Type', 'some-custom-type');
  res.send('hello');
});

app.listen(3000);

Requesting / returns Content-Type: false in the response headers.

Solution

In res.set() (lib/response.js), added a fallback to the original value when mime.contentType() returns false:

- value = mime.contentType(value)
+ value = mime.contentType(value) || value

This mirrors how res.type() already handles the same scenario:

var ct = type.indexOf('/') === -1
  ? (mime.contentType(type) || 'application/octet-stream')
  : type;

Why It Matters

• Prevents silently invalid Content-Type: false headers being sent to clients
• Makes res.set('Content-Type') behavior consistent with res.type()
• Allows custom Content-Type values that aren't in the MIME database to be used as-is

Testing

Added two new test cases to test/res.set.js:

1. should fall back to original value for unknown Content-Type — Verifies that res.set('Content-Type', 'some-custom-type') preserves the original value instead of setting 'false'
2. should resolve known Content-Type shorthands — Verifies that res.set('Content-Type', 'html') correctly resolves to text/html; charset=utf-8

Full test suite passes: 1249 passing, 0 failing.

Linked Issue

Fixes: #7034

[krzysdz]

Duplicate of #7035