Best way to retrieve data in custom api extended from wp-api

I have extended WP API plugin to provide an API which can process documents.

Routes

public function register_routes($routes)
{
    $documents_routes = array(
        '/v1/documents' = array(
            array(array($this, 'create_document'), WP_JSON_Server::CREATABLE),
        )
    );
    $new_routes = array_merge($routes, $documents_routes);
    return $new_routes;
}

callback function

public function create_document($data, $_files = null){

    if(!empty($_files)) {
        // process file
    }


    // how to retrieve 
}

Request

POST /api/v1/documents/ HTTP/1.1
Host: localhost
Cache-Control: no-cache
Postman-Token: d5277e1c-12ac-309a-034f-135173b7c7f2
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW

----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="doc_id"

3268360
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="file"; filename="PINS REST API Documentation v1.2.doc"
Content-Type: application/msword


----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="case_reference"

EN0102234
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="author"

Ghazanfar
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="stage"

1
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="category"

Application
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="type"

Document
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="description"

This is test document
----WebKitFormBoundary7MA4YWxkTrZu0gW

The Problem

I have some properties such as doc_id, case_reference, description, author, size etc which will be passed to the api along with the file.

Now the problem is I want to be able to retrieve all data if content-type:form-data is used. What is the best way to retrieve all passed data in the callback function efficiently?