Haw. I made quite the same service, curl compliant and stuff except it encrypts the file on disk upon reception (creating an AES cipher, piping the file through it while receiving it) and sending back the id of the file and the key that allows decryption of said file. If anyone is interested there's plenty of documentation, even a client that allows to take screenshots and upload them on the fly. https://up.depado.eu…
I understand it's impossible to tool the concept to work with a very simple shell one-liner, and without a further dependency like an openssl binary. I like the concept, but I'd rather have to perform a one-time install of openssl or similar and copy a 10-20 line bash script to have true security rather than your current setup that comes with a technically flawed security model.
Obviously you are aware of this, and chose the path of convenience and "good for most uses". Kudos for a clean tool!