Sending image from JS client to Java/Springboot server via fetch returns error 500

I need to send (upload) an image file from the web client (html+javascript) to the server (java/springboot) that will then store it.

On the client side I have created a form, of course

<form action="" name="missionForm" method="post" enctype="multipart/form-data" novalidate>
  <fieldset>
    <div class="field">
      <label>
        <span style="font-weight:bold">Mission name</span>
    <input data-validate-length-range="3" name="name" id="name" required="required" />
      </label>
    </div>
    <div class="field">
      <label>
    <span>Mission image (png):</span>
    <input type='file' class='optional' name='loadImage' id='loadImage' accept='image/png'>
      </label>
    </div>
 </fieldset>
</form>

The form is read below via javascript, who sends the request via fetch

document.forms[0].onsubmit = function(e){
    e.preventDefault();
   var imageFile = document.forms['missionForm']['loadImage'].files[0];
   
   const test = fetch(url+"/api/saveImage", {
    method: 'POST',
        body: imageFile,
    });
}

On the server side (Java / Springboot), I have created the following method in the Controller class

@PostMapping(value="/api/saveImage")
public void saveImage(@RequestParam("file") MultipartFile multipartFile) throws SQLException{
     String fileName = StringUtils.cleanPath(multipartFile.getOriginalFilename());
     System.out.println("filename is "+fileName);   
}

When I launch this, I get error 500 on the client.
On the server side, I get the following error “org.springframework.web.multipart.MultipartException: Current request is not a multipart request”

I do not understand why. Can anybody help? I am sure I am missing something silly here.
Thanks!

I tried “forcing” the content-type in the fetch request, but I only get a different error

   const test = fetch(url+"/api/saveImage", {
    method: 'POST',
        headers: {'Content-Type': 'multipart/form-data',},
        body: imageFile,
    }); 

and the error is “org.apache.tomcat.util.http.fileupload.FileUploadException: the request was rejected because no multipart boundary was found”