Detect the file type of a file, stream, or data

The file type is detected by checking the magic number of the buffer.

This package is for detecting binary-based file formats, not text-based formats like .txt, .csv, .svg, etc.

We accept contributions for commonly used modern file formats, not historical or obscure ones. Open an issue first for discussion.

Install

npm install file-type

This package is an ESM package. Your project needs to be ESM too. Read more. For TypeScript + CommonJS, see load-esm. If you use it with Webpack, you need the latest Webpack version and ensure you configure it correctly for ESM.

[!IMPORTANT] File type detection is based on binary signatures (magic numbers) and is a best-effort hint. It does not guarantee the file is actually of that type or that the file is valid/not malformed.

Robustness against malformed input is best-effort. When processing untrusted files on a server, enforce a reasonable file size limit and use a worker thread with a timeout (e.g., make-asynchronous). These are not considered security issues in this package.

Usage

Node.js

Determine file type from a file:

import {fileTypeFromFile} from 'file-type';

console.log(await fileTypeFromFile('Unicorn.png'));
//=> {ext: 'png', mime: 'image/png'}

Determine file type from a Uint8Array/ArrayBuffer, which may be a portion of the beginning of a file:

import {fileTypeFromBuffer} from 'file-type';
import {readChunk} from 'read-chunk';

const buffer = await readChunk('Unicorn.png', {length: 4100});

console.log(await fileTypeFromBuffer(buffer));
//=> {ext: 'png', mime: 'image/png'}

Determine file type from a stream:

import {fileTypeFromStream} from 'file-type';

const url = 'https://upload.wikimedia.org/wikipedia/en/a/a9/Example.jpg';

const response = await fetch(url);
const fileType = await fileTypeFromStream(response.body);

console.log(fileType);
//=> {ext: 'jpg', mime: 'image/jpeg'}

API

fileTypeFromBuffer(buffer, options)

Detect the file type of a Uint8Array or ArrayBuffer.

The file type is detected by checking the magic number of the buffer.

If file access is available, it is recommended to use fileTypeFromFile() instead.

Returns a Promise for an object with the detected file type:

• ext - One of the supported file types
• mime - The MIME type

Or undefined when there is no match.

buffer

Type: Uint8Array | ArrayBuffer

A buffer representing file data. It works best if the buffer contains the entire file. It may work with a smaller portion as well.

fileTypeFromFile(filePath, options)

Detect the file type of a file path.

Only available in environments where node:fs is available, such as Node.js. To read from a File, see fileTypeFromBlob().

The file type is detected by checking the magic number of the file.

Returns a Promise for an object with the detected file type:

• ext - One of the supported file types
• mime - The MIME type

Or undefined when there is no match.

filePath

Type: string

The file path to parse.

fileTypeFromStream(stream, options)

Detect the file type of a web ReadableStream.

The file type is detected by checking the magic number of the buffer.

Returns a Promise for an object with the detected file type:

• ext - One of the supported file types
• mime - The MIME type

Or undefined when there is no match.

stream

Type: Web ReadableStream

A readable stream representing file data.

[!TIP] If you have a Node.js stream.Readable, convert it with Readable.toWeb().

fileTypeFromBlob(blob, options)

Detect the file type of a Blob.

[!TIP] A File object is a Blob and can be passed in here.

It will stream the underlying Blob.

The file type is detected by checking the magic number of the blob.

Returns a Promise for an object with the detected file type:

• ext - One of the supported file types
• mime - The MIME type

Or undefined when there is no match.

import {fileTypeFromBlob} from 'file-type';

const blob = new Blob(['<?xml version="1.0" encoding="ISO-8859-1" ?>'], {
    type: 'text/plain',
    endings: 'native'
});

console.log(await fileTypeFromBlob(blob));
//=> {ext: 'txt', mime: 'text/plain'}

blob

Type: Blob

fileTypeFromTokenizer(tokenizer, options)

Detect the file type from an ITokenizer source.

This method is used internally, but can also be used for a special "tokenizer" reader.

A tokenizer propagates the internal read functions, allowing alternative transport mechanisms, to access files, to be implemented and used.

Returns a Promise for an object with the detected file type:

• ext - One of the supported file types
• mime - The MIME type

Or undefined when there is no match.

An example is @tokenizer/http, which requests data using HTTP-range-requests. A difference with a conventional stream and the tokenizer, is that it can ignore (seek, fast-forward) in the stream. For example, you may only need and read the first 6 bytes, and the last 128 bytes, which may be an advantage in case reading the entire file would take longer.

import {makeTokenizer} from '@tokenizer/http';
import {fileTypeFromTokenizer} from 'file-type';

const audioTrackUrl = 'https://test-audio.netlify.com/Various%20Artists%20-%202009%20-%20netBloc%20Vol%2024_%20tiuqottigeloot%20%5BMP3-V2%5D/01%20-%20Diablo%20Swing%20Orchestra%20-%20Heroines.mp3';

const httpTokenizer = await makeTokenizer(audioTrackUrl);
const fileType = await fileTypeFromTokenizer(httpTokenizer);

console.log(fileType);
//=> {ext: 'mp3', mime: 'audio/mpeg'}

Or use @tokenizer/s3 to determine the file type of a file stored on Amazon S3:

import {S3Client} from '@aws-sdk/client-s3';
import {makeChunkedTokenizerFromS3} from '@tokenizer/s3';
import {fileTypeFromTokenizer} from 'file-type';

// Initialize S3 client
const s3 = new S3Client();

// Initialize the S3 tokenizer.
const s3Tokenizer = await makeChunkedTokenizerFromS3(s3, {
    Bucket: 'affectlab',
    Key: '1min_35sec.mp4'
});

// Figure out what kind of file it is.
const fileType = await fileTypeFromTokenizer(s3Tokenizer);
console.log(fileType);

Note that only the minimum amount of data required to determine the file type is read (okay, just a bit extra to prevent too many fragmented reads).

tokenizer

Type: ITokenizer

A file source implementing the tokenizer interface.

fileTypeStream(webStream, options?)

Returns a Promise which resolves to the original readable stream argument, but with an added fileType property, which is an object like the one returned from fileTypeFromFile().

This method can be handy to put in a stream pipeline, but it comes with a price. Internally stream() builds up a buffer of sampleSize bytes, used as a sample, to determine the file type. The sample size impacts the file detection resolution. A smaller sample size will result in lower probability of the best file type detection.

webStream

Type: Web ReadableStream

The input stream.

options

Type: object

Supports the following options in addition to the general options:

sampleSize

Type: number\ Default: 4100

The sample size in bytes.

Example

import {fileTypeStream} from 'file-type';

const url = 'https://upload.wikimedia.org/wikipedia/en/a/a9/Example.jpg';

const response = await fetch(url);
const stream = await fileTypeStream(response.body, {sampleSize: 1024});

if (stream.fileType?.mime === 'image/jpeg') {
    // stream can be used to stream the JPEG image (from the very beginning of the stream)
}

supportedExtensions

Returns a Set<string> of supported file extensions.

supportedMimeTypes

Returns a Set<string> of supported MIME types.

Options

customDetectors

Array of custom file type detectors to run before default detectors.

For example:

import {fileTypeFromFile} from 'file-type';
import {detectXml} from '@file-type/xml';

const fileType = await fileTypeFromFile('sample.kml', {customDetectors: [detectXml]});
console.log(fileType);

mpegOffsetTolerance

Default: 0

Specifies the byte tolerance for locating the first MPEG audio frame (e.g. .mp1, .mp2, .mp3, .aac).

Allows detection to handle slight sync offsets between the expected and actual frame start. Common in malformed or incorrectly muxed files, which, while technically invalid, do occur in the wild.

A tolerance of 10 bytes covers most cases.

Custom detectors

Custom file type detectors are plugins designed to extend the default detection capabilities. They allow support for uncommon file types, non-binary formats, or customized detection behavior.

Detectors can be added via the constructor options or by directly modifying FileTypeParser#detectors. Detectors provided through the constructor are executed before the default ones.

Example adding a detector

import {FileTypeParser} from 'file-type';
import {detectXml} from '@file-type/xml';

const parser = new FileTypeParser({customDetectors: [detectXml]});
const fileType = await parser.fromFile('sample.kml');
console.log(fileType);

Available third-party file-type detectors

• @file-type/av: Improves detection of audio and video file formats, with accurate differentiation between the two
• @file-type/cfbf: Detects Compound File Binary Format (CFBF) based formats, such as Office 97–2003 documents and .msi.
• @file-type/pdf: Detects PDF based file types, such as Adobe Illustrator
• @file-type/xml: Detects common XML file types, such as GLM, KML, MusicXML, RSS, SVG, and XHTML

Detector execution flow

If a detector returns undefined, the following rules apply:

1. No Tokenizer Interaction: If the detector does not modify the tokenizer's position, the next detector in the sequence is executed.
2. Tokenizer Interaction: If the detector modifies the tokenizer's position (tokenizer.position is advanced), no further detectors are executed. In this case, the file type remains undefined, as subsequent detectors cannot evaluate the content. This is an exceptional scenario, as it prevents any other detectors from determining the file type.

Writing your own custom detector

Below is an example of a custom detector. This can be passed to the FileTypeParser via the customDetectors option.

import {FileTypeParser} from 'file-type';

const unicornDetector = {
    id: 'unicorn', // May be used to recognize the detector in the detector list
    async detect(tokenizer) {
        const unicornHeader = [85, 78, 73, 67, 79, 82, 78]; // "UNICORN" in ASCII decimal

        const buffer = new Uint8Array(unicornHeader.length);
        await tokenizer.peekBuffer(buffer, {length: unicornHeader.length, mayBeLess: true});
        if (unicornHeader.every((value, index) => value === buffer[index])) {
            return {ext: 'unicorn', mime: 'application/unicorn'};
        }

        return undefined;
    }
}

const buffer = new Uint8Array([85, 78, 73, 67, 79, 82, 78]);
const parser = new FileTypeParser({customDetectors: [unicornDetector]});
const fileType = await parser.fromBuffer(buffer);
console.log(fileType); // {ext: 'unicorn', mime: 'application/unicorn'}

/**
@param tokenizer - The [tokenizer](https://github.com/Borewit/strtok3#tokenizer) used to read file content.
@param fileType - The file type detected by standard or previous custom detectors, or `undefined` if no match is found.
@returns The detected file type, or `undefined` if no match is found.
*/
export type Detector = {
    id: string;
    detect: (tokenizer: ITokenizer, fileType?: FileTypeResult) => Promise<FileTypeResult | undefined>;
};

Abort signal

Some async operations can be aborted by passing an AbortSignal to the FileTypeParser constructor.

```js import {FileTypeParser} f