Switch to upload.php

Since `upload.php?output=gyazo` now returns the URL of the first file, switching from `blackniggers/kittens.php` should be fairly easy.

This moves the endpoint, changes the multipart form data to upload the `files` field with a filename of `image.png`, and uncomments the `Content-Type` HTTP Header lines, which will be important in future, as we'll eventually be relying on that instead of file extensions.