This PR is related to GH-20732, which is about `AuthEvent` (to delay
promting for a password), but instead adds the actual support for
encrypted attachments.
“Encrypted attachments” means that the main things are plain text.
Note that some PDF viewers, like Preview/QuickLook/Safari or Chrome,
do not support attachments at all.
Note that the file checked into the tests is the same as
`output-no-auth-event.pdf` referenced in
<https://github.com/mozilla/pdf.js/issues/20139#issuecomment-3952462166>.
Closes GH-20139.
45cdb5d3e8