What i need to do is getting the file name on file upload. I need to get the file name while uploading, write it to a cookie and read the cookie if it is set before. help please...

function NewsletterForm() {
$nameOfTheFile = ??? how to get the file name? // i need a code like $nameOfTheFile = $_FILES["file"]["tmp_name"]; on php.

return new Form($this, "NewsletterForm", new FieldSet(

// List your fields here
new FileField("ImageURL", "AddPicture"),
new EmailField("Email", "Email")....

Welcome etkinss!

You'll probably want to place this in the onAfterWrite of the File - DataObject. Is this a UserDefinedForm or your own?
If it's your own then extend the Image class and then when you write DataObject with onAfterWrite you can set your cookie. Ofcourse You might be able to access and shift scope to that image in a different way than a cookie, it would be preferable.