hat.sh is a browser-based application "that provides secure file encryption using the AES-256-GCM algorithm from WebCryptoAPI provided by your browser." The developer describes it as "fast, secure and Serverless, the app never uploads the files to the server." You can also download the application for use independent of a browser.
For ongoing encryption and more direct integration with cloud storage, consider an application like Cryptomator.
Cryptee is "a private and encrypted place for all your photos, documents, notes and more."
CryptPad is "a private-by-design alternative to popular office tools and cloud services" (e.g., Google Drive).