Add Cryptify protocol for large file transfers

[rubenhensen]

Description

When attachments exceed the email size limit, we should use the Cryptify fileshare protocol to handle large files. This also opens up an opportunity to unify the file sending and website email decrypting flows.

Current state

• File sharing via web and email decrypting via web are two separate things
• Large files cannot be sent via the plugin

Proposed approach

• When files exceed the mail size limit, transparently use Cryptify to host the encrypted file
• The email would contain a simple message/indicator that files were too large for mail but are handled securely via PostGuard
• For recipients with the plugin: files download straight into the mailbox
• For recipients without the plugin: a link to the download flow pages

Unifying web and plugin flows

The web file-sending and email-sending flows are fundamentally the same thing — an encrypted payload with metadata. The difference is just where the encrypted file lives (attachment vs Cryptify server).

A unified "PostGuard inbox" (e.g., the /decrypt page) could show:

• Encrypted email body
• Encrypted email subject
• Attached/linked files

This would also solve limitations of the current web file-sending flow, which cannot include encrypted subject/body.

[dobby-coder]

Noting overlap with #34 ("Unified PostGuard flow: email + web decryption with Cryptify"), which also proposes using Cryptify for the send path and mentions unifying the flows. The scope of this ticket (large-file handling via Cryptify) looks like a subset of #34's full unification.

Not flagging as a strict duplicate — the maintainers might want to keep them separate (large-file handling is a narrower, shippable milestone). Just cross-linking so the work doesn't get planned twice.

Dobby backlog maintenance 2026-04-16