const readChunk = require('read-chunk');
const isBmp = require('is-bmp');
const buffer = readChunk.sync('unicorn.bmp', 0, 2);

isBmp(buffer);
//=> true

const xhr = new XMLHttpRequest();
xhr.open('GET', 'unicorn.bmp');
xhr.responseType = 'arraybuffer';

xhr.onload = () => {
	isBmp(new Uint8Array(this.response));
	//=> true
};

xhr.send();