fix(eve): prefer known Telegram attachment media type over HTTP content-type

[harlley]

Closes #1217.

Description

When a photo is sent to a Telegram-channelled eve agent, createTelegramFetchFile resolves the file's media type by prioritizing the HTTP content-type header from Telegram's file-download endpoint over the media type the channel already knows from the inbound message. Telegram's file endpoint frequently responds with application/octet-stream (or no content-type) for photos, so vision models receive mediaType: "application/octet-stream" and cannot recognize the image. With an uploadPolicy: { allowedMediaTypes: ["image/*"] }, the photo is rejected before it ever reaches the model:

Telegram file rejected — photo.jpg has media type "application/octet-stream"
which is not allowed by this route. Allowed: image/*.

The fix inverts the priority so ref.mediaType (the known attachment type, e.g. image/jpeg for photos) wins, falling back to the HTTP content-type and then application/octet-stream.

How did you test your changes?

Ran the Telegram attachments unit tests against the eve package:

cd packages/eve
pnpm run build:compiled
pnpm exec vitest run --config vitest.unit.config.ts \
  src/public/channels/telegram/attachments.test.ts

• With the fix: all 4 tests pass.

• Without the fix (reverted to main): the new regression test fails with the exact production error string:

  FAIL  ... > prefers the known attachment media type over the HTTP content-type header
  Error: Telegram file rejected — photo.jpg has media type "application/octet-stream"
  which is not allowed by this route. Allowed: image/*.
  

  This confirms the test guards the regression: it fails on the old behavior and passes on the fixed one.

The regression test mocks Telegram's getFile (returning a file_path) and the file download (returning a body with Content-Type: application/octet-stream), then asserts the resolved mediaType is image/jpeg — the type the channel already knew from the inbound photo.

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted — Telegram channel serves wrong media type for photos (breaks vision models) #1217
• I ran the relevant checks from CONTRIBUTING.md (unit tests for the touched file; see commands above)
• I added tests — regression test in attachments.test.ts
• I added a changeset — .changeset/telegram-attachment-media-type.md (eve: patch)
• DCO sign-off passes for every commit (git commit --signoff) — commit is signed and GitHub-verified (SSH signing key)

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