Retrieve data passed to extended api

I have extended [WP API][1] 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?

[1]: https://ww.wp.xz.cn/plugins/json-rest-api/