bluetorodev/file_enc_dec ? reverse-engineered prompt

Build me a simple Streamlit web app that lets me upload any file, encrypt it with Fernet, and download the encrypted version right away. I also want a decrypt mode where I can upload a previously encrypted file and get the original file back, as long as the same secret key is available. If the app does not have a key yet, it should create and save one automatically in a file called secret.key. Keep the interface clean and easy to use, with clear buttons for encrypt and decrypt, file upload, and download. It should work with common file types like PDFs, images, documents, zip files, and text files. Please make it so the app runs locally with Streamlit, and use the cryptography library for the actual encryption. If needed, look up the current docs online so the implementation is up to date.