Parse, serialize, manipulate, and sniff MIME types

This package will parse MIME types into a structured format, which can then be manipulated and serialized:

``js
const { MIMEType } = require("whatwg-mimetype");

const mimeType = new MIMEType(Text/HTML;Charset="utf-8");

console.assert(mimeType.toString() === "text/html;charset=utf-8");

console.assert(mimeType.type === "text");
console.assert(mimeType.subtype === "html");
console.assert(mimeType.essence === "text/html");
console.assert(mimeType.parameters.get("charset") === "utf-8");

mimeType.parameters.set("charset", "windows-1252");
console.assert(mimeType.parameters.get("charset") === "windows-1252");
console.assert(mimeType.toString() === "text/html;charset=windows-1252");

console.assert(mimeType.isHTML() === true);
console.assert(mimeType.isXML() === false);
`

It can also determine the MIME type of a resource by sniffing its contents:

`js
const { computedMIMEType } = require("whatwg-mimetype");

const pngBytes = new Uint8Array([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]);

computedMIMEType(pngBytes).essence; // "image/png"
computedMIMEType(pngBytes, { contentTypeHeader: "image/gif" }).essence // "image/png"
computedMIMEType(pngBytes, { contentTypeHeader: "text/html" }).essence // "text/html"
`

Parsing, serialization, and sniffing are all fairly complex processes; see the specification for more details:

* parsing
* serialization
* sniffing

This package's algorithms conform to those of the WHATWG MIME Sniffing Standard, as of commit e7594d5.

APIs

$3

The MIMEType class's constructor takes a string which it will attempt to parse into a MIME type; if parsing fails, an Error will be thrown.

#### The parse() static factory method

As an alternative to the constructor, you can use MIMEType.parse(string). The only difference is that parse() will return null on failed parsing, whereas the constructor will throw. It thus makes the most sense to use the constructor in cases where unparseable MIME types would be exceptional, and use parse() when dealing with input from some unconstrained source.

#### Properties

* type: the MIME type's type, e.g. "text"
* subtype: the MIME type's subtype, e.g. "html"
* essence: the MIME type's essence, e.g. "text/html"
* parameters: an instance of MIMETypeParameters, containing this MIME type's parameters

type and subtype can be changed. They will be validated to be non-empty and only contain HTTP token code points.

essence is only a getter, and cannot be changed.

parameters is also a getter, but the contents of the MIMETypeParameters object are mutable, as described below.

#### Methods

* toString() serializes the MIME type to a string
* isHTML(): returns true if this instance represents a HTML MIME type
* isXML(): returns true if this instance represents an XML MIME type
* isJavaScript({ prohibitParameters }): returns true if this instance represents a JavaScript MIME type. prohibitParameters can be set to true to disallow any parameters, i.e. to test if the MIME type's serialization is a JavaScript MIME type essence match.

_Note: the isHTML(), isXML(), and isJavaScript() methods are speculative, and may be removed or changed in future major versions. See whatwg/mimesniff#48 for brainstorming in this area. Currently we implement these mainly because they are useful in jsdom._

$3

The MIMETypeParameters class, instances of which are returned by mimeType.parameters, has equivalent surface API to a JavaScript Map.

However, MIMETypeParameters methods will always interpret their arguments as appropriate for MIME types, so e.g. parameter names will be lowercased, and attempting to set invalid characters will throw.

Some examples:

`js
const { MIMEType } = require("whatwg-mimetype");

const mimeType = new MIMEType(x/x;a=b;c=D;E="F");

// Logs:
// a b
// c D
// e F
for (const [name, value] of mimeType.parameters) {
console.log(name, value);
}

console.assert(mimeType.parameters.has("a"));
console.assert(mimeType.parameters.has("A"));
console.assert(mimeType.parameters.get("A") === "b");

mimeType.parameters.set("Q", "X");
console.assert(mimeType.parameters.get("q") === "X");
console.assert(mimeType.toString() === "x/x;a=b;c=d;e=F;q=X");

// Throws:
mimeType.parameters.set("@", "x");
`

$3

The computedMIMEType(resource, options) function determines a resource's MIME type using the full MIME type sniffing algorithm. This includes:

* complexities around how the supplied Content-Type header or other external information interacts with (but usually does not override) the sniffing process;
* the ability to set a "no-sniff" flag, like the one used when X-Content-Type-Options: nosniff is present, which mostly (but not entirely) prevents sniffing; and
* the "Apache bug" check which occurs when the Content-Type header is one of several specific values.

That is, this doesn't just implement the rules for identifying an unknown MIME type. It gives you everything you need for a full browser-compatible MIME sniffing procedure.

#### Arguments

* resource (Uint8Array): The resource bytes.
* options.contentTypeHeader (string): The Content-Type header value, for HTTP resources.
* options.providedType (string): The MIME type from the filesystem or another protocol (like FTP), for non-HTTP resources.
* options.noSniff (boolean, default false): Whether the X-Content-Type-Options: nosniff header was present.
* options.isSupported (function, default () => true): A predicate called with a MIMEType instance to check if an image/audio/video MIME type is supported by the user agent. If it returns false, sniffing is skipped for that type.

The contentTypeHeader and providedType options will be stringified, so you could also supply a MIMEType (or a Node.js util module MIMEType).

#### Return value

A MIMEType` instance representing the computed MIME type.