index.js

import RNFS from "react-native-fs";
import { Base64 } from "js-base64";
import fileType from "file-type";

function min (a, b) {
  return a < b ? a : b;
}

function FileType(path) {
  return new Promise((resolve, reject) => {
    RNFS.stat(path)
      .then(statResult => {
        const numberBytes = min(statResult.size, 64);
        RNFS.read(path, numberBytes, 0, "base64")
          .then(fileData => {
            let convertedData = CovertBase64ToArrayBuffer(fileData);
            convertedData = new Uint8Array(convertedData);

            let type = fileType(convertedData);
            if (type === undefined || type === null) {
              let decodedData = String.fromCharCode.apply(null, convertedData);

              if (
                decodedData.startsWith("<html>") ||
                decodedData.endsWith("</html>")
              ) {
                type = { ext: "html", mime: "text/html" };
              }
            }

            resolve(type);
          })
      })
      .catch(reason => {
        reject(reason);
      });
  });
}

function CovertBase64ToArrayBuffer(data) {
  let UTF8Data = Base64.atob(data);
  let UTF8DataLength = UTF8Data.length;

  let bytes = new Uint8Array(UTF8DataLength);

  for (var i = 0; i < UTF8DataLength; i++) {
    bytes[i] = UTF8Data.charCodeAt(i);
  }

  return bytes.buffer;
}

export default FileType;