code: function dataURItoBlob(dataURI) {
    // convert base64/URLEncoded data component to raw binary data held in a string
    var byteString;
    if (dataURI.split(',')[0].indexOf('base64') >= 0)
      byteString = atob(dataURI.split(',')[1]);
    else
      byteString = unescape(dataURI.split(',')[1]);
  
    // separate out the mime component
    var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0];
  
    // write the bytes of the string to a typed array
    var ia = new Uint8Array(byteString.length);
    for (var i = 0; i < byteString.length; i++) {
      ia[i] = byteString.charCodeAt(i);
    }
  
    return new Blob([ia], {
      type: mimeString
    });
  }
docs: stackoverflow: http://stackoverflow.com/questions/4998908/convert-data-uri-to-file-then-append-to-formdata/5100158