How upload a file\image via json api?

I'm trying to upload a file or image on a D9 site via json api. The test is namely to upload the user picture on the user entity.

I've been following the documentation or read questions about, but I can't get it working.

The front-end is done with Next.js.

One the test done is the following:

The front-end next calls the next.js api server:

const filename = selectedFile.name;
const fr = new FileReader();
fr.readAsArrayBuffer(selectedFile);
fr.onload = async function () {
    const arrayBuffer = fr.result;
    if (arrayBuffer && typeof arrayBuffer !== 'string') {
        const base64String = _arrayBufferToBase64(arrayBuffer);
        const cookieSession = await getUpdateCookie();
        const res = await fetch(`/api/users-test/user?cookie=${cookieSession}&name=${filename}`, {
            method: 'POST', body: base64String,
        });
    }
}

function _arrayBufferToBase64(buffer: ArrayBuffer) {
    let binary = '';
    const bytes = new Uint8Array(buffer);
    const len = bytes.byteLength;
    for (let i = 0; i < len; i++) {
        binary += String.fromCharCode(bytes[i]);
    }
    return window.btoa(binary);
}

The api next.js server post then to Drupal:

const res = await fetch(
      `${process.env.NEXT_PUBLIC_DRUPAL_BASE_URL}/jsonapi/user/user/{userID}/user_picture`,
      {
        method: 'POST',
        headers: {
          'Content-Type': 'application/octet-stream',
          Accept: 'application/vnd.api+json',
          'Content-Disposition': `name="file"; filename="${filename}"`,
          Authorization: `Bearer ${token}`,
        },
        body: req.body,
      }
    );

I tried different ways to convert the file to "binary file data", but none work: the user picture is always a blank square with an X.

However, if I try the same call with postman, the file is correctly loaded:

So, for now, I hypothesize the problem is on the javascript code, while Drupal has been configured correctly.

I am currently having the same issue. The post goes thru perfectly in postman. the code snippet below is used to perform the post action in nextjs

//this function is used to convert the file

const readFileDataAsBase64 = (fl) => {
    const file = fl;

    return new Promise((resolve, reject) => {
        const reader = new FileReader();

        reader.onload = (event) => {
          console.log('here')
            resolve(event.target.result);
        };

        reader.onerror = (err) => {
            reject(err);
        };

        reader.readAsDataURL(file);
    });
}

//get the value of the file converted to binary
const filedata= await readFileDataAsBase64(values.files[0]);

//call the post file function to perform the post
const resfile = await postFiles(
      'http://the-url',
      filedata,
      values.files[0].name
    );

const postFiles = (url, body, filename) => {

    const myPost = async() => {
        try {
         const res = await fetch(url, {
              method: 'POST',
              body: body,
              headers: {
                'Content-Type': 'application/octet-stream',
                'Content-Disposition': `file; filename=${filename}`,
                'api-key': 'myapi-key',
              },
            });
        } catch (err){
        }
  
    } 
    postData();
  
  };

This file isn't being posted to drupal. Did anyone found a solution to properly post a file using the api