Your file is downloading, but you get it on javascript, on the data
parameter of your call because you call it with Ajax.
You use a handler – so ajax not needed here, and the most easy thing to do using javascript is that:
window.location = "https://stackoverflow.com/questions/12087040/FileDownload.ashx?parametres=22";
or with a simple link as
<a target="_blank" href="https://stackoverflow.com/questions/12087040/FileDownload.ashx?parametres=22" >download...</a>
Ah, and send the parameters via the url, you can not post them that way.
You can also read: What is the best way to download file from server