Thank you for pointing me out in the right direction so quickly. Actually I thought the “Content-Type” header was not set, in accordance with what is explicitly mentioned:

Warning: When using FormData to submit POST requests using XMLHttpRequest or the Fetch_API with the multipart/form-data Content-Type (e.g. when uploading Files and Blobs to the server), do not explicitly set the Content-Type header on the request. Doing so will prevent the browser from being able to set the Content-Type header with the boundary expression it will use to delimit form fields in the request body.

but thanks to your response I quickly found that in fact it was, and moreover it was set to "application/json" indeed.