Does File upload count against disk usage?

And if so, is there a way to offload the file upload to Google Drive instead? 

All files that are uploaded on your form will take Upload storage space of your account. 

I have checked you email address and you have only guest account. I would suggest you to sign up for a standard (free) account.

Please use the next link to sign up: https://www.jotform.com/signup 

Upload storage space for free account is  100 MB. To clear your Upload storage space you need delete the submissions with uploads on the Submission page of your account.

Please take note: Delete submissions are permanently deleted and can no longer be restored. 

I would suggest you to first download your files to your computer and then delete them. 

It is possible to integrate your form with Google Drive. Inside of this guide you can find how to do it:

https://www.jotform.com/help/192-How-to-Integrate-a-Form-with-Google-Drive 

However even if form is integrated with Google Drive data will still be stored on our servers, so you will need to delete them to clear Upload storage space.

It is also possible to prevent your submissions data from being stored in Jotform. All you need to do is to integrate our Auto Delete Submissions app with the your forms and will automatically and instantly delete your submissions from Jotform as soon as they are received. You can find the app here http://apps.jotform.com/app/auto_delete_submissions 

Hope this will help. Let us know if you need further assistance.