How to upload .xlsx in a html page

In our web application, we upload an .xlsx file with data to be processed and send to backend afterwards.

I try to add this file in the attachment section and select Media Type: multipart/form-data, but after processing the request I get this error message: Required MultipartFile parameter 'file' is not present

What am I doing wrong, and how can I create a scenario like this with Ready API?

Any help is much appreciated.