Forms :: GET_FILE_NAME Is Not Working

I am developing a form using Forms [32 Bit] Version 6.0.8.11.3 (Production). In my custom form I have given a Push_Button and my idea is to invoke a File open diaglog box when I hit the button.

So, i wrote the below code under 'when-button pressed ' triger. But, the button is not invoking the File Open diaglog box in TEST environment.

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DECLARE
XXFILENAME VARCHAR2(50);
BEGIN
XXFILENAME := get_file_name('C:', NULL,
file_filter=> 'JPEG Image (*.JPG,*.JPEG,*.JPE,*.JFIF)|*.JPG|Bitmap Image (*.bmp)|*.bmp|GIF Image (*.GIF)|*.GIF|TIFF Files (*.tif)|*.tif|All Files (*.*)|*.*|');
END;