Add the ability to register third-party extensions

core.d.ts

@@ -369,6 +369,10 @@ declare namespace core {
 	```
 	*/
 	function stream(readableStream: ReadableStream): Promise<core.ReadableStreamWithFileType>
+
+	type Detection = (tokenizer: ITokenizer, fileType?: core.FileTypeResult) => Promise<core.FileTypeResult | undefined>;
+
+	function addDetection(detection: Detection): void;
 }
 
 export = core;

core.js

@@ -10,6 +10,8 @@ const supported = require('./supported');
 
 const minimumBytes = 4100; // A fair amount of file-types are detectable within this range
 
+const detections = [_detectFileType];
+
 async function fromStream(stream) {
 	const tokenizer = await strtok3.fromStream(stream);
 	try {
@@ -66,15 +68,28 @@ async function _checkSequence(sequence, tokenizer, ignoreBytes) {
 
 async function fromTokenizer(tokenizer) {
 	try {
-		return _fromTokenizer(tokenizer);
+		return detectFileType(tokenizer);
 	} catch (error) {
 		if (!(error instanceof strtok3.EndOfStreamError)) {
 			throw error;
 		}
 	}
 }
 
-async function _fromTokenizer(tokenizer) {
+async function addDetection(detection) {
+	detections.push(detection);
+}
+
+async function detectFileType(tokenizer) {
+	let fileType;
+	for (const detection of detections) {
+		fileType = await detection(tokenizer, fileType);
+	}
+
+	return fileType;
+}
+
+async function _detectFileType(tokenizer) {
 	let buffer = Buffer.alloc(minimumBytes);
 	const bytesRead = 12;
 	const check = (header, options) => _check(buffer, header, options);
@@ -1400,7 +1415,8 @@ const fileType = {
 	fromStream,
 	fromTokenizer,
 	fromBuffer,
-	stream
+	stream,
+	addDetection
 };
 
 Object.defineProperty(fileType, 'extensions', {

index.d.ts

@@ -23,5 +23,7 @@ export {
 	fromTokenizer,
 	extensions,
 	mimeTypes,
-	stream
+	stream,
+	Detection,
+	addDetection
 } from './core';

readme.md

@@ -268,6 +268,16 @@ Returns a set of supported file extensions.
 
 Returns a set of supported MIME types.
 
+### FileType.addDetection(detection)
+
+Register a detection function. This function will be called after `file-type` and other detections complete.
+
+The `detection(tokenizer, fileType)` method has two arguments:
+- [tokenizer](https://github.com/Borewit/strtok3#tokenizer)
+- `fileType`, which is `undefined` or the return value of a detected file type.
+
+If the detector returns `undefined`, the `tokenizer.position` should be 0. That allows other detectors to parse the file. 
+
 ## Supported file types
 
 - [`jpg`](https://en.wikipedia.org/wiki/JPEG)