Getting "422 Unprocessable Entity" error when positing stringified body from node.js to Python FastAPI

Question

I have this request, on my node.js application :

return new Promise((resolve, reject) => {
      const options = {
        hostname: 'py', <== docker container
        port: 8000,
        path: '/resume',
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
        },
        body: JSON.stringify({
          file
        })
      }
  
      const req = http.request(options, (res) => {
        const body = [];
        console.log('STATUS: ' + res.statusCode);
        console.log('HEADERS: ' + JSON.stringify(res.headers));
        res.setEncoding('utf8');
        res.on('data', (chunk) => {
          console.log('BODY: ' + chunk);
          body.push(chunk);
        });

        resolve(body);
      });
  
      req.on('error', (error) =>{
        console.log('problem with request: ' + error.message);
        reject(error)
      });

      req.end();
    })

And this, on my python app, that I copied from this topic => https://stackoverflow.com/a/65141532/14317753

@app.post("/resume")
async def resume(request: Request):
    da = await request.form()
    da = jsonable_encoder(da)
    print('file', da)
    return da

But unfortunately, da is an empty string.

Obviously, I've tried everything : changing the GET request to POST and putting the get file param into a base64 file body param, using the request.json() and the request.body() instead of request.form() for the fixes I remember.

Thanks for you help.