sS hjs_fileUploadBase64Encoder() {
  ret hscript([[
    var fileConverted = false;
    
    function submitWithFileConversion(form) {
      if (fileConverted) return true;
      fileConverted = true;
      var file = $('#fileUploader')[0].files[0];
      if (file == null) return true;
      var reader = new FileReader();

      // delay form submit until reader is done
      reader.onloadend = function () {
        var b64 = reader.result.replace(/^data:.+;base64,/, '');
        $("#f_file_base64").val(b64);
        console.log("Got base64 data: " + b64.length);
        setTimeout(function() {
          $(form).submit();
        }, 1000);
      };

      reader.readAsDataURL(file);
      return false;
    }
  ]]);
}

Began life as a copy of #1029740