HTTP onion sites do not give a popup warning when submiting form data to non-onion HTTP sites
In vanilla firefox, HTTPS site with a form that submits to an HTTP site will put up a pop-up box warning the user they are about to send their data unencrypted across the network. Tor Browser follows this behaviour, but it also needs to do so when submitting data from an HTTP onionsite to a vanilla HTTP site.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information