The ciphertext was sealed to your capsule's public key. Only the private key you hold can open it, and it opens right here. Nothing you paste leaves this page.
How it works: the file is an NDC2 container sealed to your capsule public key. This page parses it, uses your private key to HPKE-unwrap the per-file key, and AES-GCM-decrypts the body, all with Web Crypto in this tab. The server that stored the ciphertext cannot do any of this; it never held your key.