JS/JSON file upload immutable dictionary is empty - ajax

Question

I know, similar questions had been asked before.
All questions I found concerns upload within a form and need a submit button. This I don't want. I want upload to start after file selection is made (I'm aware of security risks).
Why is the contend of the file dictionary not send?
The button itself is created by JS:
downLoadBtn = document.createElement('INPUT');
downLoadBtn.id = 'downloadBtn';
downLoadBtn.type = 'file';
downLoadBtn.onchange = function(){upload()}
function upload(){
file = document.getElementById('downloadBtn')
console.log(file['files']['0'])
$.ajax({
type: "POST",
url: '/upload',
dataType : 'json',
data: JSON.file,
success: function (response){
console.log(response);
}
});
}
python:
#app.route('/upload', methods = ['POST'])
def upload():
if request.method == 'POST':
data = request.files
print(data)
return jsonify(data)
What I'm doing wrong?

Answer 1

You pass JSON.file as the data parameter which is not defined in the code you posted, it should be the file tat was selected.
Since your server side code expects multipart from data, you'll have to use a FormData object.
function upload(){
file = document.getElementById('downloadBtn')
console.log(file['files']['0'])
var fd = new FormData();
fd.append('file', file['files']['0']);
$.ajax({
type: "POST",
url: '/upload',
dataType : 'json',
data: fd,
contentType: false,
processData: false,
success: function (response){
console.log(response);
}
});
}