Convert Base64 string to Blob.

base64_to_blob.js

function base64ToBlob(b64String, callback) {
  var contentType = b64String.split(',')[0].split(':')[1].split(';')[0];
  var byteCharacters = atob(b64String.split(',')[1]);
  var sliceSize = 512;

  var byteArrays = [];
  for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
    var slice = byteCharacters.slice(offset, offset + sliceSize);
    var byteNumbers = new Array(slice.length);
    for (var i = 0; i < slice.length; i++) {
      byteNumbers[i] = slice.charCodeAt(i);
    }
    var byteArray = new Uint8Array(byteNumbers);
    byteArrays.push(byteArray);
  }

  var blob = new Blob(byteArrays, { type: contentType });
  callback(blob);
};