How can I send a multipart/post-data POST request via a resource controller?

By trying to create a custom REST API through a resource controller via a custom module in Drupal 9, I would like to be able to send my data from a multipart/form-data Content-Type in order to pass files along with my textual data.

To be more precise, I have a Drupal content type with different text fields and with the possibility for the user to add attachments.

Currently I can only use a JSON format (via raw from Postman) for textual data which is not practical for sending files from my API making it impossible because the Content-Type application/json does not allow it.

As soon as I switch to multipart/form-data from Postman, my api gives me the following error: "No route found that matches "Content-Type: multipart/form-data; boundary=[...]"

Here is a snippet of my code:

class FormuelResource extends ResourceBase {

  /**
   * Requete POST pour la création d'un formuel
   * 
   * @param array $data
   *
   * @return \Drupal\rest\ResourceResponse
   *
   * @throws \Symfony\Component\HttpKernel\Exception\HttpException
   */
  public function post(array $data) {

    $msgRecap = ['message' => 'Vous venez de créer le formuel suivant : '.$data["title"]. '. Un email de confirmation vient de vous être envoyé.'];
  
    $typeContenu  = 'formuel';
    $node = Node::create(['type' => $typeContenu]);
    $node->uid = 1;
    $node->promote = 0;
    $node->sticky = 0;

    // Champs du formuel

    // Identification de l'agent
    $email = $data["identifiant"].'@mail.com';

    $node->field_uuid = $data["uid"];
    $node->field_identifiant = $data["identifiant"];
    $node->field_email = $email;
    $node->field_prenom = $data["prenom"];
    $node->field_nom= $data["nom"]; 
    $node->field_num_tel = $data["tel"];
    $node->field_site = $data["site"];
    $node->field_pieces_jointes = $data["pj"]; // files
    $node->field_api = true;

    $node->field_app_ou_materiel = $data["app"];
    $node->title=  $data["title"];
    $node->body = $data["body"];
        
    $node->save(); // Sauvegarde dans la base du formuel
    $nid = $node->id(); // Récupération nid

    [...]

    $response = new ResourceResponse($msgRecap);

    $response->addCacheableDependency($msgRecap);
    return $response;

   }

}

Which gives me at the level of my request in JSON on Postman:

URL : http://localhost/api/formuel/add?_format=json

raw :

{
    "uid": "123456",
    "identifiant": "john.doe",
    "prenom": "John",
    "nom": "DOE",
    "tel": "0623456789",
    "site": "My site",
    "app": 4,
    "title": "Test title node via API",
    "body": "Content node via API"
}

Response (success) :

{
    "message": "Vous venez de créer le formuel suivant : Test Formuel API 115. Un email de confirmation vient de vous être envoyé."
}

But going through the form-data with all of my keys and inserting files gives me an error :

Thank you in advance for your reply :)