Try setting the value after the HTML element is created. For example:

Code:
var fileNameId = Ext.id();            
var jsonStrId= Ext.id();
var form = Ext.DomHelper.append(document.body, {
    tag: 'form',
    method: 'post',
    action: 'BounceBackServlet',
    children: [
        {tag: 'input', type:'hidden', name: 'fileName', id: fileNameId},
        {tag: 'input', type:'hidden', name: 'jsonStr',    id: jsonStrId}
    ]
});
document.body.appendChild(form);

var fileNameEl = document.getElementById(fileNameId);
fileNameEl.value = fileName;

var jsonStrEl = document.getElementById(jsonStrId);
jsonStrEl.value = body;

form.submit();
document.body.removeChild(form);