Rename file when import

Here is script which import a mp4 file from my "d" drive.

{
var imgDir = ("d:");
var newAmt = 1;
if (!app.project) { app.newProject(); }
var myItemCollection = app.project.items;
var myComp = app.project.activeItem;
var myProperty;
function processFolder(theFolder)
{

for (var j=4; j < 5; j++)
{
var importOpts = new ImportOptions( new File(theFolder + "aff"+".mp"+j) );
var importImg = app.project.importFile (importOpts);
var myImg = myComp.layers.add(importImg, );
}
}
processFolder(imgDir);
}

it import a "aff.mp4" file from my directory. But i want to rename the "aff.mp4" file when it import. is it possible? Please help me...