fix(cli): accept every file type the relay accepts on upload - #7581

Problem

buzz upload file and buzz messages send --file accept only jpeg, png, gif, webp
and mp4 (ALLOWED_MIMES in crates/buzz-cli/src/client.rs). The relay accepts much
more: buzz-media::validation refuses only active web content and native
executables, and stores anything without magic bytes as application/octet-stream
(its own test asserts plaintext is accepted that way).

The practical effect is that an agent cannot hand a document back to the people it
works with. Ask an agent for a written deliverable and it writes the file, then
cannot attach it:

$ buzz upload file --file plan_v2.md
{"error":"user_error","message":"unsupported file type: application/octet-stream","retryable":false}

We watched an agent try .md, copy it to .txt, then .json, and finally give up
and paste an 18 KB document into the message body.

Change

• upload_mime_for() replaces the allow-list with the relay's deny-list (mirroring
  BLOCKED_FILE_MIME_TYPES): text, PDFs, archives and the rest upload, while
  executables and active web content are still refused client-side.
• BlobDescriptor gains filename, set from the uploaded path, and
  build_imeta_tag emits filename <name> — the NIP-92 field the Desktop already
  writes and reads for generic attachments.
• buzz messages send --file renders a non-image/video attachment as
  [name](url) instead of ![image](url), matching the Desktop.

buzz upload file now prints filename in its JSON descriptor when known
(omitted otherwise) — additive, existing parsers are unaffected.

Size caps are unchanged (50 MB, 500 MB for video); the constant's doc comment was
reworded because it is no longer image-only.

Tests

upload_mime_tests in client.rs: plaintext sniffs to application/octet-stream,
PNG magic bytes to image/png, a PE header is refused; the imeta tag carries the
filename and mime.

Manual test

buzz upload file --file notes.md
buzz messages send --channel <uuid> --file notes.md 'here you go'

The Desktop renders a named download link and the file opens.

Note

One line in crates/buzz-acp/src/base_prompt.md tells agents that --file exists —
without it they keep pasting documents into the message body, which is the behaviour
this fixes. Happy to split that out if you would rather keep the prompt separate.

🤖 Generated with Claude Code