1. first sort out the requirements, others upload to your server, only upload documents? Do you need additional information?
2. secondly, you need to design the interface: do you need a unified directory? For example, how to implement this requirement through URL Rewrite if URL can be www.xxx.com/api/postFile,; whether the communication method is GET or POST (but you obviously need to use POST); to handle additional messages sent by users; correct / error messages returned by users after calling the API, etc.)
3. all of the above are solved one by one, and then sorted out into interface documents.
4. online joint tuning of the function.

api the same as the domain name of api caller ? If it is inconsistent, consider the cross-domain issue.

suppose the domain name of api is http://api.com, and the domain name of the caller is http://get.com.

I defined a api as follows:

// api
//  cors 
// 
header('Access-Control-Allow-Origin: http://get.com');
header('Access-Control-Allow-Credentials: true');

class UploadFile {
    public function upload(){
        //  ...
    }
}

call http://api.com/UploadFile/upload and then add the required parameters.

caller js :

var apiUrl = 'http://api.com/UploadFile/upload';
// 
var formData = new FormData();
    // 
    formData.append('file' , file);
var xhr = new XMLHttpRequest();
    xhr.open('post' , apiUrl , true);
    xhr.onreadystatechange = function(){
        ...
    };
    xhr.send(formData);