Improves file upload testing documentation

[cheike569]

This change will improve the File Upload Testing Documentation, as this has been hidden in the "File Upload" Section when it should be under the Testing section. It was not clearly pointed out how to only pass form fields with the extra option instead of the body (like one would assume) Also, it was not clearly pointed out that Symfony's FormDataPart Class won't work (like described at https://symfony.com/doc/current/http_client.html).