Hi there - with WordPress.com you don't need to upload the verification file. All you need to do is head over to Tools > Available Tools in your dashboard:
https://kabbageuk.wordpress.com/wp-admin/tools.php
Scroll down to the section called "Website Verification Services" - and enter the code Google supplies in the field next to "Google Webmaster Tools."
It looks like you already managed to do that, so kudos to you!