forcing non-standard extention

I work with "device independent bitmap" (DIB) files, which PsCS2 recognizes as BMP files. Opening them with a script is no problem but Ps wants to save them as filename.bmp files instead of filename.dib. The code I use for saving is:

DocRef.saveAs( new File( resultsFolder + "/" + fileNames[fileIndex] + "_Hm-and-Mt.dib" ), bmpSaveOptions, true );

and bmpSaveOptions are defined as:

var bmpSaveOptions = new BMPSaveOptions();
bmpSaveOptions.alphaChannels = false;
bmpSaveOptions.depth = BMPDepthType.EIGHT;
bmpSaveOptions.osType = OperatingSystem.WINDOWS;
bmpSaveOptions.rleCompression = false;

Does anyone know if I can force the extension'dib'?

TIA :)