Normally entire PDFs are encrypted (or not).
But it is also possible to only encrypt attachments.
It is then also possible to *only* prompt for a password when the user opens
them.
In the existing flow, prompting for passwords happens because things are decrypted.
A specific error is thrown, caught, and the user is prompted.
To keep this flow working, this PR changes to decrypting attachments on demand,
instead of eagerly.
This sounds logical: to not read attachments on startup.
I’ve extensively tested this, not only with regular attachments, but also with outline items
and attachments in annotations.
This PR builds on GH-21234.
It’s an alternative to the naïve GH-20732.
Closes GH-20049.
4db9e45b8c