Laravel api - how to upload file

I am building a web service for mobile application using Laravel. I need to let the users to upload images. I am using passport package. In developing environment I created a function store as follows

public function store(Request $request)
{
    return $request;
}

and using Firefox poster I post contents. These are the parameters

The problem is the function store always returns empty array while I expect it to return information about the file. What is wrong here?

thanks

via Chebli Mohamed