Aren't existing encryption tools more flexible for sending out copies via those methods? e.g. by making more cipher suites available? Or by allowing you to encrypt with the intended recipients' public keys, so you don't need to worry about distributing the passphrase securely? And might allow better integration with password managers, as the OS would recognise the files as being encrypted?
I was thinking of editing my comment to add "Or be able to support filetypes other than HTML", but I suppose in our glorious "everything is the browser" future, filetypes other than HTML are as anachronistic as 8" floppy disks.
FWIW, you can use data URLs to include any type of file in your HTML, and with <a download='filename' href='data:…'> browsers will allow you to save the file.
