Files are encrypted client-server-side with AES-256-CBC using a key derived from your passphrase via PBKDF2-HMAC-SHA256 (600,000 iterations + 8-byte random salt). The output is byte-identical to:

openssl enc -aes-256-cbc -pbkdf2 -iter 600000 -salt -in input -out output.enc

You will need the exact same passphrase to decrypt. There is no recovery; lose the passphrase and the file is gone.