Receiving AJAX submitted Javascript's FormData object

If you send down form data, you should use the content-type multipart/form-data not application/json. If Jason.DecodeError is being thrown Phoenix probably thinks it’s json, which means the content type header is likely the culprit.

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.