fix(buzz-cli): expand upload MIME allowlist to match relay's generic file path

[shawnhank]

Summary

• buzz upload file / buzz messages send --file rejected PDFs, Office documents, archives, and plain text client-side before the request ever reached the relay — ALLOWED_MIMES only listed 4 image types + video/mp4.
• The relay's generic file-upload path (validate_file_content in buzz-media) already accepts anything not on its BLOCKED_FILE_MIME_TYPES denylist (active web content, executables) — the CLI's stricter allowlist was purely a client-side gap. Verified server-side behavior by reading crates/buzz-media/src/validation.rs directly.
• Per both issues' suggested fix, mirrors the relay's blocklist approach instead of maintaining a second, divergent allowlist: recognized image/video MIME types keep their existing dedicated handling (size caps, relay pipelines); everything else passes the client-side check unless it's a recognized image/video/audio type (which the relay's generic path also fails closed, since audio has no dedicated pipeline yet) or on the denylist.
• Adds a third size tier (100 MB, MAX_FILE_BYTES) for the generic path, matching the relay's default_max_file_bytes. Previously non-video files were checked against MAX_IMAGE_BYTES (50 MB) regardless of type.
• Slightly improved the rejection error message, since the old wording ("unsupported file type") was misleading for types the relay's denylist has always accepted.

Fixes #2963, fixes #3083.

Test plan

• cargo test -p buzz-cli — 352 passed, including two new tests: upload_accepts_pdf_via_generic_file_path (PDF magic bytes now succeed against a mock relay) and upload_rejects_recognized_audio_type (audio is still rejected client-side, before any network call, since the relay's generic path fails it closed too)
• cargo clippy -p buzz-cli --all-targets -- -D warnings — clean
• cargo fmt -p buzz-cli