Custom Form Field Name

(function() {
 new MultipleUpload('#demo', {
 uploadUrl: '/api/upload',
 multiple: true,
 fieldName: 'attachment'
 });
})();

Default: The form field is named file. Set fieldName to match your server-side parameter name.

The name the file arrives under

fieldName defaults to 'file'. It is the multipart form field the file is sent as, so it has to match what your server reads: request.files['file'], IFormFile file, $_FILES['file'], and so on.

The symptom when it is wrong

The upload succeeds — a 200, a completed progress bar, a queue row that looks right — and the server finds no file. Nothing errors, because as far as HTTP is concerned the request was fine; your handler simply looked under a name that was not there. If uploads “work” but nothing lands, check this before anything else.