There's an example on how to encrypt the files before sending them to the server:

  # Encrypt files with password using gpg 
  $ cat /tmp/hello.txt|gpg -ac -o-|curl -X PUT --upload-file "-" https://transfer.sh/test.txt 
  
  # Download and decrypt 
  $ curl https://transfer.sh/1lDau/test.txt|gpg -o- > /tmp/hello.txt