{
xtype: 'uploadbutton',
                    plugins: [{
                        ptype: 'ux.upload.window',
                        title: 'Upload',
                        width: 520,
                        pluginId: 'uploadWin',
                        modal: true // <<------------
                    }]
}

{
  xtype: 'uploadbutton',
  listeners: {
    beforestart: function(uploader) {
      if (isValidSomehow) return true;
      else return false;
    }
  }
}