Setting a filename when upload a blob


We generate a PDF document locally in JS. This PDF doc is uploaded to knack using this code:

var file = new window.File([blob], 'filename.pdf')
var form = new window.FormData()
form.append('files', file, 'kursusbevis.pdf')


The problem is, when the PDF file is download, it gets a auto-generated filename instead of "filename.pdf".

How do we make Knack keep the filename?