Opt-in PDF attachments through the existing upload pipeline

[alex-furman]

PDF rejection is deliberate current behavior, not a claimed regression. Issue #222 identifies the missing PDF-reading capability and leaves PDF handling as future work. A number of agent runtimes can supply their own bounded native PDF reader, but the shared upload processor rejects the original before that reader can access it.

Would maintainers accept a focused, operator-controlled opt-in for delivering PDF originals through the existing workspace upload path?

Revised proposal:

• Add ALLOW_PDF_UPLOADS, disabled by default. Only explicit true enables PDF delivery; unset, false and invalid values retain rejection. Document and wire the setting into all three Compose profiles (development, production and hosted).
• Apply the policy to declared PDFs and detected/header-identified PDFs, including generic octet-stream uploads.
• When enabled, preserve original bytes and existing size limits, filename sanitization, MIME checks, audit attribution, session storage and cleanup. Other MIME mismatches remain rejected.
• Keep parsing, OCR, longer retention and agent-to-agent routing outside core. Operators should enable delivery only for deployments whose agents have suitable document-reading tools. Format detection does not establish validity or safety.

We considered a separate document-only webhook relay, but it would introduce another ingress/credential/storage/retry boundary. An explicit, default-off setting appears smaller and easier to maintain.

The opt-in candidate now passes 146 affected tests, including default-off, explicit-on, invalid configuration and disguised/preamble PDF regressions. Independent review reproduced and helped close a missing-detector bypass; the final reproducer confirms rejection with zero workspace writes. No remaining review blocker. The earlier unconditional design is superseded.

Related #499 covers a broader per-agent public-channel file policy; this does not fulfill or close it. If this smaller option fits the roadmap, could it receive an accepted scoped issue or agreed subtask for a dev-targeted PR under CONTRIBUTING.md?

[vybe]

Thanks for revising this — the default-off version is much easier to say yes to. Accepted as scoped: ALLOW_PDF_UPLOADS, disabled unless explicitly true, covering declared and detected PDFs, with the existing size limits, filename sanitization, MIME checks, audit attribution and cleanup unchanged. Parsing, OCR and retention changes stay out of core, as you proposed.

I've opened #3871 for it — please target dev with Fixes #3871. In the PR, please document the setting in the user docs (who should turn it on, and that delivery is not validation), and keep the disguised-PDF and zero-write regression tests you mentioned. Agreed that this neither fulfils nor closes #499.