Pico-optimization: `.toString` defaults to `utf8` which is "expensive". If you specify `ascii`, it's a slightly cheaper operation. (I remember that from Max Ogden's talk at nodeconf.eu!)

**&lt;aside&gt;**
I wrote that then wondered – is it true that mimetypes can only contain ascii characters? So I dug up the spec for naming, [RFC 4288, section 4.2](https://tools.ietf.org/html/rfc4288#section-4.2). It says,

```
Type and subtype names MUST conform to the following ABNF:

    type-name = reg-name
    subtype-name = reg-name

    reg-name = 1*127reg-name-chars
    reg-name-chars = ALPHA / DIGIT / "!" /
                    "#" / "$" / "&" / "." /
                    "+" / "-" / "^" / "_"
```

And looking up `ALPHA` in the ABNF spec ([RFC 5234, section B.1. Core Rules](http://tools.ietf.org/html/rfc5234#appendix-B.1)), it is specifically `/[a-zA-Z]/`.

All that's to say, my curiosity led me on an interesting journey and I spent more time thinking about this than could ever possibly be saved by making the change, which is probably unnecessary :)
**&lt;/aside&gt;**