How can I delete images/files to save storage space?

How can I delete images/files uploaded as part of a submission in a form, without deleting the submission itself? My storage quota is currently over the limit and I would like to reduce it. I am not seeing a facility to do this; seems like you do this to force people to upgrade to the next payment tier...?

I checked your account and your File Upload Storage is over the limit. Upon checking it further, your File Upload Storage at the moment is 518.18 MB. Also, your limit is only 100 MB for the Storage plan. 

Unfortunately, you need to delete the submission with the attachment file to clear your File Upload Storage. 

To delete the submission, please follow this guide: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data