fix(res.set): prevent Content-Type from being set to 'false' for unknown types

[cyphercodes]

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.

Root Cause

In res.set() (response.js), the Content-Type branch does:

if (field.toLowerCase() === 'content-type') {
  if (Array.isArray(value)) {
    throw new TypeError('Content-Type cannot be set to an Array');
  }
  value = mime.contentType(value)  // can return false!
}
this.setHeader(field, value);

When mime.contentType(value) returns false, the value false is passed to setHeader, which coerces it to the string "false".

Solution

This fix checks if mime.contentType() returns false and falls back to the original value instead:

var ct = mime.contentType(value);
if (ct !== false) {
  value = ct;
}

This is consistent with how res.type() handles unknown types.

Changes

• Modified lib/response.js to handle the case when mime.contentType() returns false
• Added test case in test/res.set.js to verify the fix

Fixes #7034

[krzysdz]

Duplicate of #7035