Best Way to Convert js object

I try to Upload Files with Ajax Post and form.serialize, but my req.body looks like this:

    {
    _uploadFilesMessage: '',
    mailgroupIDS: '',
    'field[0][fileID]': '0',
    'field[0][filename]': 'img_nature_wide27.jpg',
    'field[0][filetype]': '2',
    'field[0][selTag]': '322444370236',
    'field[1][fileID]': '1',
    'field[1][filename]': 'img_snow_wide10.jpg',
    'field[1][filetype]': '2',
    'field[1][selTag]': '322444370236'
  }

but i need it like this:

{
  _uploadFilesMessage: '',
  mailgroupIDS: '',
  field: [
    {
      fileID: '0',
      filename: 'img_nature_wide27.jpg',
      filetype: '2',
      selTag: '322444370236'
    },
    {
      fileID: '1',
      filename: 'img_snow_wide10.jpg',
      filetype: '2',
      selTag: '322444370236'
    }
  ]
}

What is the most efficient way to convert the req.body ?