How to "save as" a document using scripts

I want to use scripts to allow me to automatize creation of name badges. I have a PSD template of the badge that includes a slot for the photo and a text field for the name. The aproach I took in my script is simple:
1. Prompt the user to select the source folder for the photos.

2. For each folder in the folder, do the following:
    - resize the photo

    - copy the background layer of the photo

    - close the document and focus back on the PSD template

    - place the copied photo as a new layer

    - translate it to a certain position so it fits into the photo slot of the template

    - change the text in the text layer to the name of the photo we placed (which is the actual name of the person)

3. Save the photo as jpg with some given save options in a certain output folder.

Step 1 and 2 work just as needed. However, when I have to save the badge, I get a really strange error saying:

Here's the whole code (in JS) :

var sourceFolder = Folder.selectDialog ("Selecteaza folderul cu pozele: ");  //prompt the user to select the source folder
var files = new Array();
files = sourceFolder.getFiles ();  //put the files from the folder inside an array    var startRulerUnits = app.preferences.rulerUnits;  //save the initial units used    var startTypeUnits = app.preferences.typeUnits;     app.preferences.rulerUnits = Units.PIXELS;  //change the units to pixels so the translate command will work as expected    app.preferences.typeUnits = TypeUnits.PIXELS;
var mainDoc = app.documents.getByName ("Ecuson.psd"); //this is the name badge template
var so = new JPEGSaveOptions(); //the save options i'll use with the save command
so.embedColorProfile = true;
so.formatOptions = FormatOptions.STANDARDBASELINE;
so.matte = MatteType.WHITE;
so.quality = 10;
for(var i = 0; i<files.length; ++i){ //loop thru each photo in the folder    var img = app.open(files[i]); //open the image    var saveFile = new File("C:/Users/Eugen/Desktop/salvate/" + String.valueOf(i+1)); //this is the file where I'll save the jpg file    var nume = img.name.replace(".jpg", ''); //this is the name of the image I just opened. I remove the jpg extension so only the name is left    app.preferences.rulerUnits = startRulerUnits; //I change the units back t their original value as for this phase I need to use CM    app.preferences.typeUnits = startTypeUnits;    img.resizeImage (3, 4, 300); //resize the image to 3x4 cm at 300 dpi    app.preferences.rulerUnits = Units.PIXELS; //change the units to pixels again    app.preferences.typeUnits = TypeUnits.PIXELS;    img.backgroundLayer.copy(); //copy the background (and only) layer of the image    img.close(SaveOptions.DONOTSAVECHANGES); //close the document without saving changes    app.activeDocument = mainDoc; //focus back on the badge template    var pLayer = mainDoc.paste(); //paste the layer I just copied into the template    pLayer.translate (-314, 33); //place the new layer where I need it to be    mainDoc.layers.getByName("Name").textItem.contents = nume; //change the name on the badge to the name of the original file    mainDoc.saveAs (saveFile, so, true, Extension.LOWERCASE); //this is ehere I have issues...    }    app.preferences.rulerUnits = startRulerUnits; //change back to centimeters    app.preferences.typeUnits = startTypeUnits;

I guess it can only be a problem with the saveFile agument, as I can save the jpg manually from photoshop.

PS: using CS6

Thanks!